Linear viscoelastic relaxation modulus of polydisperse poly(dimethylsiloxane) melts containing unentangled chains

This work analyzes the relationship between the shear relaxation modulus of entangled, linear and flexible homopolymer blends and its molecular weight distribution (MWD) when a fraction of the sample contains chains with molecular weight M lower than the effective critical molecular weight between entanglements Mc_eff. This effective critical parameter is defined in terms of the critical molecular weight between entanglements M_c of the bulk polymer that forms the physical network and the effective mass fraction Wc_eff of the unentangled chains. In the terminal zone of the linear viscoelastic response, the double reptation mixing rule for blended entangled chains and a modified law for the relaxation time of chains in a polydisperse matrix are considered, where the effect of chains with M<Mc_eff is included. Although chain reptation with contour length fluctuations and tube constraint release are still the relevant mechanisms of chain relaxation in the terminal zone when the polydispersity is high, it is found that the presence of a fraction of molecules with M<Mc_eff modifies substantially the tube constrain release mode of chain relaxation. In this sense, a modified relaxation law for polymer chains in a polydisperse entangled melt that includes the effect of the MWD of unentangled chains is proposed. This law is validated with rheometric data of linear viscoelasticity for well-characterized polydimethylsiloxane (PDMS) blends and their MWD obtained from size exclusion chromatography. The short time response of PDMS, which involves the glassy modes of relaxation, is modeled by considering Rouse diffusion between entanglement points of chains with M>Mc_eff. This mechanism is independent from the MWD. The unentangled chains with M<Mc_eff occluded in the polymer network also follow Rouse modes of relaxation although they exhibit dependence on the MWD.     

Na-feldspar (F) and kaolinite (K) system at high temperature: Resulting phase composition,micro-structural features and mullite-glass Gibbs energy of formation,as a function of F/K ratio and kaolinite crystallinity

The system Na-feldspar (F) and kaolinite (K) was investigated at temperatures of interest in ceramic applications (1200–1280 °C) to study the effects of F/K ratios by weight and crystallinity degree of kaolinite on the final product, micro-structural features and mullite-glass Gibbs energy of formation (ΔG^eff). Mullite and glass are the dominant phases; in general, the higher the temperature, the larger the former. An F/K increase promotes the formation of glass and secondary mullite, appearing along with the primary one. ΔG^eff was modelled by α(T) × (F/K)² + β(T) × F/K + γ(T), α, β and γ being linear functions of temperature whose coefficients were determined by fitting the ΔG^eff-theoretical to the ΔG^eff-obtained from the measured phase compositions. ΔG^eff is less affected by temperature than by F/K, whose increase shifts equilibrium towards glass phases. The ΔG^eff-curves for ordered and disordered kaolinite intersect one another at F/K ～0.5, a ratio close to that used in industrial practice.     

Role of reptation in the rate of crystallization of polyethylene fractions from the melt

The theory of polymer crystallization with chain folding is extended to include the effect of reptation in the melt on the rates of crystallization G_I and G_II in régimes I and II. The result is that the pre-exponential factors for G_I and G_II contain a factor $\text{1}\text{n}$, Where n is the number of monomer units in the pendant chain being reeled onto the substrate by the force of crystallization; n is proportional to the molecular weight. The predicted fall in growth rate with increasing molecular weight is found experimentally in nine polyethylene fractions M_z=2.65 × 10⁴ to M_z=2.04 × 10⁵, corresponding to n_z=1.90 × 10³ to 1.45 × 10⁴. The data on these fractions are analysed to find the reptation or ‘reeling’ rate r and the substrate completion rate g. The values g_nuc～0.5/n_z cm s^?1 and r_nuc～21/n_z cm s^?1 at 400K are obtained from the data in conjunction with nucleation theory adapted to account for reptation assuming a substantial degree of regular folding. These results are consistent with a melting point in the range of ～142° to ～145°C. (The analysis using T°_m(∞)=145°C gives values of such quantities as σ σ_e and α that are quite similar to those deduced in earlier studies.) An estimate of g (denoted g_expt) that is independent of the molecular details of nucleation theory gives g_expt～0.4/n_z cm s^?1 and r～17/n_z cm s^?1 at 400K. Calculations of the reptation rate from r_1,2 = (force of crystallization ÷ friction coefficient for reptation in melt), where the friction coefficient is determined from diffusion data on polyethylene melts, leads to r_1,2～17/n_z to 34/n_z cm s^?1 at at 400K, or g_1,2～0.4/n_z to 0.8/n_z cm s^?1. The conclusion is that the reptation rate characteristic of the melt is fast enough to allow a significant degree of adjacent re-entry or ‘regular’ folding during substrate completion at the temperature cited, and that the substrate completion process is governed jointly by the activation energy for reptation $\text{Q1}{}_{\mathrm{D}}$ and the work of chain folding q. The nucleation theory and the friction coefficient theory approaches are compared, and the formulations found to be essentially equivalent; the ‘reeling’ rate r is found to be proportional to $\text{(}\text{1}\text{n}\text{)A}{}_0\text{(Δf)v}{}_0\text{exp}\text{[?}\text{(Q1}{}_{\mathrm{D}}\text{+q)}\text{RT}\text{]}$, where v₀ is a frequency factor, and A₀(Δf) is the force of crystallization on the pendant chain. The data analysis on the fractions confirms the detailed applicability of régime theory. The growth rate theory presented allows the possibility that the growth front may be microfaceted in régime I.     

Effect of soft segment molecular weight on tensile properties of poly(propylene oxide) based polyurethaneureas

Influence of soft segment molecular weight and hard segment content on the morphology, thermomechanical and tensile properties of homologous polyurethaneurea copolymers based on narrow molecular weight poly(propylene oxide)glycol (PPG) oligomers were investigated. A series of polyurethaneureas with hard segment contents of 12–45% by weight and PPG number average molecular weights <M_n> of 2000 to 11,800 g/mol were synthesized and characterized structurally by SAXS and mechanically by DMA and stress strain analysis. Bis(4-isocyanatocyclohexyl)methane and 2-methyl-1,5-diaminopentane were used as the diisocyanate and the chain extender respectively. All copolymers displayed microphase separation by SAXS and DMA. The critical entanglement molecular weight (M_e) of PPG is reported to be around 7700 g/mol. Our mechanical results suggest that when copolymers possess similar hard segment contents and are compared to those based on soft segments with number average molecular weights (M_n) greater than M_e, they generally displayed higher tensile strengths and particularly lower hysteresis and creep than those having soft segment molecular weights below M_e. These results imply that soft segment entanglements in thermoplastic polyurethaneureas may provide a critical contribution to the tensile properties of these copolymers – particularly in the range where the soft segment content is dominant.     

Role of chain entanglements on fiber formation during electrospinning of polymer solutions: good solvent, non-specific polymer-polymer interaction limit

Chain entanglements are one of many parameters that can significantly influence fiber formation during polymer electrospinning. While the importance of chain entanglements has been acknowledged, there is no clear understanding of how many entanglements are required to affect/stabilize fiber formation. In this paper, polymer solution rheology arguments have been extrapolated to formulate a semi-empirical analysis to explain the transition from electrospraying to electrospinning in the good solvent, non-specific polymer-polymer interaction limit. Utilizing entanglement and weight average molecular weights (M_e, M_w), the requisite polymer concentration for fiber formation may be determined a priori, eliminating the laborious trial-and-error methodology typically employed to produce electrospun fibers. Incipient, incomplete fiber formation is correctly predicted for a variety of polymer/solvent systems at one entanglement per chain. Complete, stable fiber formation occurs at ≥2.5 entanglements per chain.     

Stereocomplex crystallization and spherulite growth behavior of poly(l-lactide)-b-poly(d-lactide) stereodiblock copolymers

The non-isothermally and isothermally crystallized stereodiblock copolymers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with equimolar l-lactyl and d-lactyl units and different number-average molecular weights (M_n) of 3.9 × 10³, 9.3 × 10³, and 1.1 × 10⁴ g mol⁻¹, which are abbreviated as PLLA-b-PDLA copolymers, contained only stereocomplex crystallites as crystalline species, causing higher melting temperatures of the PLLA-b-PDLA copolymers compared to those of PLLA homopolymers. In the case of non-isothermal crystallization, the cold crystallization temperatures of the PLLA-b-PDLA copolymers during heating and cooling were respectively lower and higher than those of PLLA homopolymers, indicating accelerated crystallization of PLLA-b-PDLA copolymers. In the case of isothermal crystallization, in the crystallizable temperature range, the crystallinity (X_c) values of the PLLA-b-PDLA copolymers were lower than those of the PLLA homopolymers, and were susceptible to the effect of crystallization temperature in contrast to that of homopolymers. The radial growth rate of the spherulites (G) of the PLLA-b-PDLA copolymers was the highest at the middle M_n of 9.3 × 10³ g mol⁻¹. This trend is different from that of the PLLA homopolymers where the G values increased monotonically with a decrease in M_n, but seems to be caused by the upper critical M_n values of PLLA and PDLA chains as in the case of PLLA/PDLA blends (in other papers), above which homo-crystallites are formed in addition to stereocomplex crystallites. The disturbed crystallization of PLLA-b-PDLA copolymers compared to that of the PLLA/PDLA blend is attributable to the segmental connection between the PLLA and PDLA chains, which interrupted the free movement of those chains of the PLLA-b-PDLA copolymers during crystallization. The crystallite growth mechanism of the PLLA-b-PDLA copolymers was different from that of the PLLA/PDLA blend.     

Dependence of zero-shear viscosity and steady-state compliance on molecular weight between entanglements for ethylene-cycloolefin copolymers

Dynamic viscoelastic properties of a series of cyclic olefin copolymers have been investigated. The specimens differ in total molecular weight as well as molecular weight between entanglements. The angular frequency (ω) dependence curves of dynamic storage and loss moduli (G′ and G″, respectively) of the specimens have shown that G′ ∝ ω² and G″ ∝ ω in the terminal region, and a plateau region at high ω. On the basis of the experimental results, the dependence of total molecular weight as well as molecular weight between entanglements has been examined for zero-shear viscosity (η₀) and steady-state compliance (J_e). It is shown that for the melts of the copolymers in the entangled regime, M_w being the weight-average molecular weight and M_e the molecular weight between entanglements. The steady-state compliance J_e for the melts scales with M_e and M_w as .     

A correlation between critical end-to-end distance for entanglements and molecular chain diameter of polymers

The critical molecular weight M_c of 36 flexible and semirigid polymers has been studied. A unique correlation between the critical end-to-end distance 〈R_c〉 for entanglements and the average polymer chain diameter D is found. This correlation is discussed in the light of the reptation concept.     

Coordination assemblies of CdII/ZnII/CoII with the 3-(pyridin-4-yl) benzoate tecton: Structural diversity and properties

Three novel coordination polymers{[CdL₂(HL)(H₂O)]·(H₂O)}_n (1), [Zn₂(OH)L₃]_n (2) and [CoL₂(H₂O)]_n (3) have been hydrothermally synthesized from M(NO₃)₂·nH₂O (M^II = Cd^II, Zn^II and Co^II) and an unsymmetrical tecton 3-(pyridin-4-yl) benzoic acid (HL). All complexes show interesting structural patterns, namely, unprecedented 1D double-stranded supramolecular clasp for 1, novel (3,5,6)-connected helical tubular double layer for 2, 2-fold interpenetrating cds network for 3. The fluorescent and thermal properties of complexes 1, 2 and/or 3 have also been investigated.     

Spherulite growth of l-lactide copolymers: Effects of tacticity and comonomers

The spherulite growth behavior and mechanism of l-lactide copolymers, poly(l-lactide-co-d-lactide) [P(LLA-DLA)], poly(l-lactide-co-glycolide) [P(LLA-GA)], and poly(l-lactide-co-ε-caprolactone) [P(LLA-CL)] have been studied using polarization optical microscopy in comparison with poly(l-lactide) (PLLA) having different molecular weights to elucidate the effects of incorporated comonomer units. The incorporation of comonomer units reduced the radius growth rate of spherulites (G) and increased the induction period of spherulite formation (t_i), irrespective of the kind of comonomer unit. Such effects became remarkable with the content of comonomers. At a crystallization temperature (T_c) of 130 °C, the disturbance effects of comonomers on the spherulite growth decreased in the following order: d-lactide>glycolide>ε-caprolactone, when compared at the same comonomer unit or reciprocal of averaged l-lactyl unit sequence length (l_l). The t_i estimation indicated that the glycolide units have the lowest disturbance effects on the formation of spherulite (crystallite) nuclei. The PLLA having the number-average molecular weight (M_n) exceeding 3.1×10⁴ g mol⁻¹ showed the transition from regime II to regime III at T_c=120 °C, whereas PLLA with the lowest M_n of 9.2×10³ g mol⁻¹ crystallized solely in regime III kinetics and the copolymers excluding P(LLA-DLA) with 3% of d-lactide units crystallized solely according to regime II kinetics. The nucleation and front constant for regime II and III [K_g(II), K_g(III), G₀(II), and G₀(III), respectively] estimated with each (not with a fixed for high-molecular-weight PLLA) decreased with increasing the amount of defects per unit mass of the polymer for crystallization, i.e. with increasing the comonomer content and the density of terminal group through decreasing the molecular weight.     

RAFT polymerization of diacrylate derivatives having different spacers in dilute conditions

Reversible addition-fragmentation chain transfer (RAFT) polymerization of four divinyl monomers, 1,4-butanediol diacrylate (BDDA) and three poly(ethylene glycol) diacrylates (PEGDAs), were investigated under dilute conditions ([M] = 0.2-0.05 mol/L). RAFT polymerization of BDDA using a dithiocarbamate-type chain transfer agent (CTA) afforded soluble polymers, whereas a cross-linked product was obtained by conventional radical polymerization. The monomer concentration, the nature of the CTA, and the CTA/initiator ratio were found to affect the polymerization behavior and structure of the resulting polymers, which is attributed to the relative propensities for intermolecular propagating/cross-linking reactions and intramolecular cyclization. RAFT polymerizations of three PEGDAs (PEG258DA, average M_n = 258; PEG575DA, average M_n = 575; PEG700DA, average M_n = 700) having different lengths of PEG spacers (n = 3, 10, 13, respectively) were also conducted under dilute conditions. Water-soluble polymers were synthesized by one-step RAFT polymerization of PEGDAs having longer spacers (n = 10 and 13), whereas RAFT polymerization of PEGDA (n = 3) afforded polymers soluble in organic solvents. The product obtained by RAFT polymerization of PEGDA (n = 10) showed a characteristic thermoresponsive property, lower critical solution temperature (LCST), in aqueous solution.     

Synthesis of poly(N-vinyl carbazole)-based block copolymers by sequential polymerizations of RAFT–ATRP

Poly(N-vinylcarbazole) (PNVK) is one of the extensively studied photoconductive polymers because of its wide ranges of applications. Through the reversible addition-fragmentation chain transfer/macromolecular design via the interchange of xanthates (RAFT/MADIX) polymerizations, in this study we investigated the syntheses of PNVK-based block copolymers (BCPs) with styrene (St) and methyl methacrylate (MMA). A variety of difunctional haloester-xanthate inifers were prepared and subjected to sequential polymerizations through RAFT and ATRP. In the presence of small amounts of bromoxanthate inifers, the ¹H NMR spectra showed nearly complete consumption of the NVK monomer, but without formation of PNVK. The bromoxanthate inifer could act as acidic moieties that protonated the highly basic NVK monomer. Through ¹H NMR and MALDI-TOF spectroscopic analyses, the structures of byproducts were indentified and a plausible mechanism for their formation was proposed. Alternatively, RAFT/MADIX polymerizations of NVK with two chloroxanthate inifers S-[1-methyl-4-(6-chloropropionate)ethyl acetate] O-ethyl dithiocarbonate and S-[1-methyl-4-(6-chloroisobutyrate)ethyl acetate] O-ethyl dithiocarbonate) provided first-order kinetic plots and well-controlled PNVK-Cl MIs (M_n ≈ 6000–40,000; M_w/M_n < 1.35). Using a suitable ATRP-initiating groups and optimization of the reaction conditions, the BCPs PNVK-b-PSt (M_n ≈ 4900–12,800; M_w/M_n < 1.5) and PNVK-b-PMMA (M_n ≈ 46,000–100,000; M_w/M_n < 1.35) were obtained.     

Iodine transfer dispersion polymerization (dispersion ITP) with CHI3 and reversible chain transfer catalyzed dispersion polymerization (dispersion RTCP) with GeI4 of styrene in supercritical carbon dioxide

Submicrometer-sized, polystyrene (PS) particles with controlled molecular weight distribution (MWD), were successfully obtained directly as powder state by iodine transfer dispersion polymerization (dispersion ITP) and reversible chain transfer catalyzed dispersion polymerization (dispersion RTCP) in supercritical carbon dioxide (scCO₂) for the first time. These dispersion polymerizations proceeded similarly reaching 80% conversion in 21 h. In the dispersion ITP, the number-average molecular weight (M_n) nonlinearly increased with the conversion, which were always higher than theoretical values, and the MWD at each conversion was comparatively narrow (M_w/M_n = 1.5–1.7) throughout polymerization. In the dispersion RTCP, M_n also nonlinearly increased with increasing conversion and M_w/M_n values were in the range of 1.3–1.5, which were lower than those of the dispersion ITP. In chain extension tests in bulk systems, the degrees of livingness of PS prepared by the dispersion ITP and the dispersion RTCP in scCO₂ systems were, respectively, estimated to be 56% and 48%. From these results, while more investigation is necessary, it was concluded that both polymerizations with scCO₂ proceeded in a partly controlled manner.     

Room temperature living cationic polymerization of styrene with HX-styrenic monomer adduct/FeCl3 systems in the presence of tetrabutylammonium halide and tetraalkylphosphonium bromide salts

Living cationic polymerization of styrene was achieved with a series of initiating systems consisting of a HX-styrenic monomer adduct (X = Br, Cl) and ferric chloride (FeCl₃) in conjunction with added salts such as tetrabutylammonium halides (nBu₄N⁺Y⁻; Y⁻ = Br⁻, Cl⁻, I⁻) or tetraalkylphosphonium bromides [nR′₄PBr; R′ = CH₃CH₂-, CH₃(CH₂)₂CH₂-, CH₃(CH₂)₆CH₂-] or tetraphenylphosphonium bromide [(C₆H₅)₄PBr] in dichloromethane (CH₂Cl₂) and in toluene. Comparison of the molecular weight distributions (MWDs) of the polystyrenes prepared at different temperatures (e.g., −25 °C, 0 °C and 25 °C) showed that the polymerization is better controlled at ambient temperature (25 °C). The polymerization was almost instantaneous (completed within 1 min) and quantitative (yield ∼100%) in CH₂Cl₂. In CH₂Cl₂, polystyrenes with moderately narrow (M_w/M_n ∼ 1.33-1.40) and broad (M_w/M_n ∼ 1.5-2.4) MWDs were obtained respectively with and without nBu₄N⁺Y⁻. However, in toluene, the MWDs of the polystyrenes obtained respectively with and without nBu₄N⁺Y⁻/nR′₄P⁺Br⁻ were moderately narrow (M_w/M_n = 1.33-1.5) and extremely narrow (M_w/M_n = 1.05-1.17). Livingness of this polymerization in CH₂Cl₂ was confirmed via monomer-addition experiment as well as from the study of molecular weights of obtained polystyrenes prepared simply by varying monomer to initiator ratio. A possible mechanistic pathway for this polymerization was suggested based on the results of the ¹H NMR spectroscopic analysis of the model reactions as well as the end group analysis of the obtained polymer.     

Synthesis and characterization of low-refractive-index fluorinated silsesquioxane-based hybrids

Novel low-refractive-index silsesquioxane-based hybrids were synthesized via hydrolytic condensation of fluorinated triethoxysilane precursors, which were prepared by reacting 3-aminopropyl triethoxysilane with acrylates containing fluoroalkyl groups, 1H,1H,5H-octafluoropentyl acrylate (OFPA) and 2,2,2-trifluoroethyl acrylate (TFEA). The hydrolytic condensations proceeded as a homogeneous system in acetone in the presence of aqueous HF solution (3.2%) at 30 °C. The products were soluble in a variety of organic solvents, including CHCl₃, THF, and acetone, but were insoluble in hexane and water. The structures of the products were confirmed by ¹H NMR, ¹³C NMR, ¹⁹F NMR, and FT-IR spectroscopy. The low polydispersities and reasonable molecular weights of the resulting fluorinated silsesquioxanes (M_n = 5800, M_w/M_n = 1.01; and M_n = 4700, M_w/M_n = 1.04 for the OFPA- and TFEA-based products, respectively) were confirmed by size-exclusion chromatography. Scanning force microscope (SFM) measurements indicated the formation of spherical hybrids having relatively narrow size distributions (average particle diameter < 3.0 nm) without aggregation. The sizes of the hybrids were also confirmed by X-ray diffraction (XRD). The refractive indexes of the TFEA- and OFPA-based silsesquioxane hybrids were 1.43 and 1.40, respectively. These results indicate the formation of novel fluorinated silsesquioxane-based hybrids having good solubility, narrow size distribution, and a low refractive index. Co-condensation of the TFEA- and OFPA-based triethoxysilane precursors affords a series of fluorinated hybrids whose refractive index and various other properties can be manipulated by varying the composition of the feed.     

Impact of the cation field strength on physical properties and structures of alkali and alkaline-earth borosilicate glasses

The impact of the cation field strength (CFS) of the glass network-modifier cations on the structure and properties of borosilicate glasses (BS) were examined for a large ensemble of mixed-cation (R/2)M₍₂₎O–(R/2)Na₂O–B₂O₃–KSiO₂ glasses with M⁺ ={Li⁺, Na⁺, K⁺, Rb⁺} and M²⁺ ={Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺} from four series of {K, R} combinations of K = n(SiO₂)/n(B₂O₃) = {2.0, 4.0} and R =[n(M₍₂₎O) ?+ ?n(Na₂O)]/n(B₂O₃) = {0.75, 2.1}. Combined with results from La³⁺ bearing glasses enabled the probing of physical-property variations across a wide CFS range, encompassing the glass transition temperature (T_g), density, molar volume and compactness, as well as the hardness (H) and Young's modulus (E). We discuss the inferred composition–structure/CFS–property relationships. Each of T_g, H, and E revealed a non-linear dependence against the CFS and a strong T_g/H correlation, where each property is maximized for the largest alkaline-earth metal cations, i.e., Sr²⁺ and Ba²⁺, along with the high-CFS La³⁺ species. The ¹¹B MAS NMR-derived fractional BO₄ populations decreased linearly with the average M^z+/Na⁺ CFS within both K–0.75 glass branches, whereas the NBO-rich K–2.1 glasses manifested more complex trends. Comparisons with results from RM₂O–B₂O₃–KSiO₂ glasses suggested no significant “mixed alkali effect”.     

Control of methyl methacrylate radical polymerization via Enhanced Spin Capturing Polymerization (ESCP)

The nitrone mediated polymerization of methyl methacrylate (MMA) via the enhanced (termination) spin capturing polymerization (ESCP) process is made possible via the addition of small amounts of styrene (between 5 and 10 vol.%) to the reaction mixture. Efficient control over the molecular weight between 7000 and 57,000 g mol⁻¹ (at 60 °C) yields macromolecules that feature a mid-chain alkoxyamine functionality and are rich in methyl methacrylate. The collated kinetic and molecular weight data allow for a deduction of the spin capturing constant, C_SC, in the range between 0.15 and 0.30. During the ESCP process, the number average molecular weight, M_n, of the formed mid-chain functional polymer is constant up to high monomer to polymer conversions (i.e. 80%). The high degree of alkoxyamine mid-chain functionality present in the generated polymeric material is evidenced via a subsequent nitroxide-mediated polymerization process employing the formed ESCP polymer, indicating a chain extension from 37,700 to 118,000 g mol⁻¹ with a concomitant reduction in polydispersity (from 2.3 to 1.5).     

Long terminal linear alkyl group as internal crystallization accelerating moiety of poly(l-lactide)

Poly(l-lactide) (PLLA) polymers having terminal n-alkyl groups with a wide variety of lengths (C₀–C₂₂) were synthesized by ring-opening polymerization of l-lactide in the presence of coinitiators of l-lactic acid (C₀), 1-hexanol (C₆), 1-dodecanol (C₁₂), and 1-docosanol (C₂₂) and their segmental mobility and non-isothermal and isothermal crystallization behavior were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffractometry (WAXD). Glass transition and cold crystallization temperatures of melt-quenched samples during heating decreased with an increase in the length of terminal n-alkyl groups. The enhanced PLLA segmental mobility and hydrophobic interaction-based accelerated PLLA nucleation by the presence of terminal long n-alkyl groups should have caused the accelerated non-isothermal and isothermal crystallization of PLLA segments traced by cold crystallization temperature during heating and by radial growth rate of spherulites, respectively. The crystallization accelerating effect became higher with the length of terminal n-alkyl groups. The effects of the length of terminal n-alkyl group on the crystalline growth mechanism of PLLA at the lowest crystallizable temperature was insignificant, whereas the effects of the length of terminal n-alkyl group on the nucleation mechanism of PLLA chains were significant and insignificant for PLLA having M_n of 6–7 × 10³ of 2 × 10⁴ g mol⁻¹, respectively. WAXD measurements revealed that the transition crystallization temperature at which crystalline modification changes from δ-form to α-form was affected by the length of terminal n-alkyl group for PLLA having M_n of 6–7 × 10³ g mol⁻¹, but was not altered by the length of terminal n-alkyl group for PLLA having M_n of 2 × 10⁴ g mol⁻¹.     

Preparation of thin film alloys with electrodeposition from heterotrinuclear M1–M2–M1 complexes (M1: NiII, CuII; M2: CuII, CoII, ZnII, CdII)

M₁ ^II–M₂ ^II–M₁ ^II type linear complexes (where M₁ = Ni²⁺, Cu²⁺ and M₂ = Cu²⁺, Zn²⁺, Co²⁺ and Cd²⁺ were prepared and dissolved in dimethyl sulfoxide. They were then electrodeposited on mild steel surfaces by the use of rotating disc electrodes. The deposition potentials were determined from cyclic voltammetric i-E scans. The metal films deposited on mild steel surfaces were characterized with atomic absorption spectrometer (AAS), X-ray fluorescence (XRF) and ion chromatography (IC) methods. Although the stoichiometric quantity of M₂ metal cation was half of the M₁ metal cation in the complex, the amount of M₂ metal deposited upon the surface was markedly greater. The amount of M₂ ion deposited on the surface was found to increase with the hardness of the ion. The X-ray diffraction (XRD) patterns showed that the excessively deposited metal on the surface was in metallic form as well as alloy. The size of the deposited film particles was investigated by the use of atomic force microscope (AFM) technique and the particles were observed to be bigger than nanoparticle size.     

Cryogenic magnetic properties and magnetocaloric effects (MCE) in B-site disordered RE2CuMnO6 (RE = Gd,Dy, Ho and Er) double perovskites (DP) compounds

In this paper, a detailed investigation with respect to the structural, cryogenic magnetic properties and magnetocaloric performances of RE₂CuMnO₆ (RE = Gd, Dy, Ho and Er) double perovskite (DP) compounds has been performed. All the RE₂CuMnO₆ compounds are confirmed to B-site disordered and crystallized in the GdFeO₃-type structure (Pnma space group, N 62, oP20). The magnetic transition temperatures (T_M) are found to be ~7.5 K for Gd₂CuMnO₆, ~12.1 K for Dy₂CuMnO₆, ~12.2 K for Ho₂CuMnO₆, and ~3.6 K for Er₂CuMnO₆, respectively. Moreover, for checking the magnetocaloric performances several vital parameters including -ΔS_M^max (peak value of magnetic entropy change, -ΔS_M), TEC (3) (temperature averaged -ΔS_M) and RCP (relative cooling powers) are evaluated to be 7.84, 7.73 J/kgK and 151.1 J/kg for Gd₂CuMnO₆, 5.69, 5.59 J/kgK and 180.9 J/kg for Dy₂CuMnO₆, 7.12, 7.05 J/kgK and 192.4 J/kg for Ho₂CuMnO₆, as well as 9.92, 9.60 J/kgK and 195.9 J/kg for Er₂CuMnO₆ under the magnetic field change ΔH = 5 T, respectively.