A spectroscopic analysis of conformational distortion in the α′ phase of poly(lactic acid)

Simulations in conjunction with Raman scattering were performed to determine the conformational distortion of the α′ form of poly(lactic acid) (PLA). Based on packing constraints of an overall helical structure, the chain conformation can only be approximated by slight deviations from a tg’t chain conformation. Models were generated with different types of random departures from the expected dihedral angles. Raman active vibrations in the 700 cm⁻¹ region that are sensitive to changes in chain conformation were analyzed. The model that showed the best fit to the experimental data contains a majority (80%) of tg’t-10₃ sequences with randomly distributed tg’t-3₁ units. Departure of the O-C_α dihedral angle was also proved. The mechanism of order formation was established. As PLA chains crystallize at low temperatures, only the α′ phase with a disordered helix can be formed which at elevated temperatures can transform to the more ordered α phase. The conformation distortion is then due to the inability for some chains to complete the transition to 10₃ helicity, thus resulting in the formation of the distorted α′ structure.     

Molecular Orientation and Mechanical Properties of Poly(Vinyl Alcohol) Fibers

Abstract

A number of poly(vinyl alcohol) fibers with different draw ratios was characterized by measuring the birefringence, crystalline orientational order, crystallinity, tensile strength, and modulus. The birefringence, tensile strength and modulus increased with increasing draw ratio whereas the crystallinity and crystalline order parameters remained constant within narrow limits. The increase in birefringence has to be attributed solely to an increase in chain orientation in the amorphous phase of the semicrystalline fiber. The tensile strength and modulus are therefore directly related to the chain orientation in the amorphous phase. With the aid of a simple two-phase model it was found that the modulus of the amorphous phase in its disordered conformation was 4.8 GPa. The intrinsic birefringence of the amorphous phase was found to be 79 × 10^?3, i.e. much higher than the value obtained for the crystalline phase (52 × 10^?3). When this value was used in calculations, it was found that the order parameter of the amorphous phase increased from around 0.1 for a draw ratio of 1 to approximately 0.6 for a draw ratio of 5, whereas the order parameter of the crystalline phase was close to 1 for all draw ratios.     

Stretching-induced crystallinity and orientation of polylactic acid nanofibers with improved mechanical properties using an electrically charged rotating viscoelastic jet

An electrospinning technique with an additional centrifugal field was employed to prepare polylactic acid (PLA) nanofibers. The results indicated that combining the strong stretching force of the additional centrifugal field and an electrostatic field can align the PLA polymer chains parallel to the nanofiber axis, producing PLA nanofibers with superior crystalline features, molecular orientation and conformation as well as good mechanical properties. The optimal stretching force for an electrically charged rotating viscoelastic jet was determined by high-speed videography and the analysis of dimensionless groups (i.e., the Re, We, and Oh numbers). The elastic modulus of PLA nanofibers with a crystallinity degree of 37% and a dichroic ratio (D) of 1.89 was measured at 2.64 GPa and 3.30 GPa by atomic-force microscopy (AFM) and nanoindenter experiments, respectively. Furthermore, the optimal PLA nanofibers produced by the proposed method can potentially be applied for the reinforcement of composites.     

Study of crystalline and amorphous phases of biodegradable poly(lactic acid) by advanced thermal analysis

The qualitative and quantitative thermal analysis of biodegradable poly(lactic acid) PLA is presented. The glass transition, melting process, and heat capacity of a semi-crystalline poly(lactic acid) are studied utilizing the differential scanning calorimetry and temperature-modulated DSC. The mobile amorphous fraction, W_a degree of crystallinity, W_c and rigid-amorphous fraction, W_RAF were estimated depending on the thermal history of semi-crystalline PLA. From qualitative thermal analysis, the glass transition of rigid-amorphous phase was observed as a broadening from the changes of heat-flow-rate between mobile glass transition temperature and melting temperature. The amount of the rigid-amorphous fraction (RAF) was evaluated from W_RAF = 1−W_c − W_a and graphically was presented as the result of a deflection from the linearity of the dependence of the change of degree of mobile amorphous phase (W_a) vs. the degree of crystalline fraction (W_c) for semi-crystalline PLA with different thermal history. The degree of crystallinity of semi-crystalline samples of PLA can be discussed in terms of a two- or three-phase model. In contrast, the quantitative thermal analysis of the experimental apparent heat capacity of semi-crystalline PLA did not show any appearance of RAF in the examples of analyzed samples. The experimental heat capacity of PLA was analyzed in reference to the solid and liquid equilibrium heat capacities of poly(lactic acid) found in the ATHAS Data Bank.     

Conformational transition characterization of glass transition behavior of polymers

The conformational transition behavior of polymer in the amorphous state has been investigated through molecular dynamics simulations across the glass transition temperature (T_g). We find that the conformational transition, a localized and short time dynamics feature, crosses over different barrier heights when the system transforms from the molten state into the glass state and the barrier height in the glass state is markedly lower than that above T_g. In addition to the overall transition behavior, the specific transitions between the rotational isomeric states (RIS) g⁺, t⁻, t⁺ and g⁻ are also investigated in detail. The populations of these specific transitions undergo considerable changes when the temperature decreases; meanwhile, the larger transition rates of the ending torsions get diminished. Besides the rate, the rotation degrees of the dihedrals during the transitions also change their distributions tremendously through T_g, below which most of the larger transition angles (50-100°) were inhibited remaining those sharply around 30°. This possibly explains why below T_g the conformational transition process has a lower effective barrier.     

Conformational development of polylactide films induced by uniaxial drawing

Uniaxially drawn polylactide (PLA) films were prepared using various draw ratios (2, 3, 4 and 4.5) at a constant draw rate and temperature. It was confirmed that the conformational structure of the PLA films prepared using the uniaxial drawing process was composed of the α′‐phase form. The conformational structure deformation of α′‐phase PLA films, according to the various draw ratios used, was investigated in terms of strain‐induced crystalline behavior and molecular orientation analysis. It is of utmost importance to confirm the α′‐phase structural deformation caused by the uniaxial drawing process because it directly relates to the characteristics of PLA films. The conformational structure deformations of the α′‐phase, created by uniaxial drawing, led to improved mechanical properties, as evident from mechanical testing and dynamic mechanical analyses results. © 2013 Society of Chemical Industry     

Structure, phase transition and electric properties of poly(vinylidene fluoride-trifluoroethylene) copolymer studied with density functional theory

The geometry, energy, internal rotation, vibrational spectra, dipole moments and molecular polarizabilities of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) of α- and β-chain models were studied with density functional theory at B3PW91/6-31G(d) level and compared with those of the poly(vinylidene fluoride) (PVDF) homopolymer. The chain length and the trifluoroethylene (TrFE) concentration were examined to discuss the copolymer chain stabilities, chain conformations and electric properties. The asymmetrical internal-rotation potential energy curve shows that the angles for the g and g′ conformations in the α-chain (tg and tg′) models are 53° and −70°, respectively, and the β-chain (ttt) conformation is a slightly distorted all-trans plane with dihedral angle at 177°. The energy differences, E_β − E_α(g) and E_β − E_α(g′), between the β- and the α-conformation are 2.1 and 7.8 kJ/mol, respectively. These values are smaller than that in PVDF (8.4 kJ/mol), suggesting that the β-conformation in the copolymer will be more stable than in PVDF. The energy barriers for β → α(g) and β → α(g′) transitions are 16.2 and 5.8 kJ/mol, respectively. The former is almost twice of the energy barrier in PVDF by 8.2 kJ/mol and the latter is slightly smaller (by 2.4 kJ/mol) than that in PVDF. The respective energy barriers for α(g) → β and α(g′) → β transitions are 18.3 and 13.6 kJ/mol compared with the value 16.3 kJ/mol in PVDF. The asymmetrical energy barriers may be one of the reasons for the copolymers with 0.5-0.6 (mole fraction) VDF exhibiting complicated phase transition behavior. The conformation of α-chain P(VDF-TrFE) exhibits from a helical (containing higher TrFE) to a nearly beeline (containing lower TrFE). This behavior is different from that in the PVDF and the nearly beeline conformation might be responsible for the increasing crystallizability. The helical might also be associated with the complicated phase transition behavior and the larger lattice strain in the P(VDF-TrFE)s with higher TrFE concentration. The energy difference per monomer unit between the β- and α-chain decreases with increasing TrFE content. The ideal β-chain is curved with a radius of about 30 Å, which is similar to that in PVDF. The chain curvature and the TrFE content will affect the dipole moment contribution per monomer. The chain length and TrFE content will not significantly affect the mean polarizability. The calculations indicated that there are some additional characteristic vibrational modes that may be used in identification of the α- or β-phase P(VDF-TrFE)s with different TrFE contents.     

Extrusion drawn amorphous and semicrystalline poly(ethylene terephthalate): 3. Linear thermal expansion analysis

Thermal expansion analysis of uniaxially-oriented poly(ethylene terephthalate) (PET) films has been carried out from 123 K up to the PET glass transition temperature, T_g. The films are prepared by solidstate co-extrusion, from different premorphologies (amorphous, 33% and 50% crystalline), to draw ratios (EDR) up to 4.4, over a wide temperature range (T_ext). The coefficients of linear thermal expansion exhibit anisotropy: normal to the draw direction (α_⊥) it increases, whereas along the draw direction (α_∥) it always decreases with draw, independent of the initial morphology. Results are interpreted by treating PET as a simple two-phase composite structure. Tie-molecules occurring in the amorphous domains and bridging adjacent crystallites have a major influence. At EDR=4.4, a significant number of taut tiemolecules are developed, resulting in α_∥ becoming negative (α^max_∥=?1.0×10^?5°C^?1) at temperatures below ambient. This appears to be the first report of a negative α for a polymer of relatively low crystallinity. Temperature for extrusion draw significantly affects α. Normal to the draw direction, α_⊥ decreases with T_ext whereas α_∥ increases. The results show the thermal expansion of PET depends primarily on orientational effects. Only in the absence of anisotropy does per cent crystallinity have a dominant influence. In addition, the TMA thermograms display sharp transitions which are attributed to irreversible shrinkage of the oriented films. The shrinkage temperature (T_s) shows a strong dependence on both orientation and crystallinity, and it is discussed in association with T_g.     

The Role of Birch Tar in Changing the Physicochemical and Biocidal Properties of Polylactide-Based Films

The objective of this study was to produce bactericidal polymer films containing birch tar (BT). The produced polymer films contain PLA, plasticiser PEG (5% wt.) and birch tar (1, 5 and 10% wt.). Compared to plasticised PLA, films with BT were characterised by reduced elongation at break and reduced water vapour permeability, which was the lowest in the case of film with 10% wt. BT content. Changes in the morphology of the produced materials were observed by performing scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis; the addition of BT caused the surface of the film to be non-uniform and to contain recesses. FTIR analysis of plasticised PLA/BT films showed that the addition of birch tar did not change the crystallinity of the obtained materials. According to ISO 22196: 2011, the PLA film with 10% wt. BT content showed the highest antibacterial effect against the plant pathogens A. tumefaciens, X. campestris, P. brassicacearum, P. corrugata, P. syringae. It was found that the introduction of birch tar to plasticised PLA leads to a material with biocidal effect and favourable physicochemical and structural properties, which classifies this material for agricultural and horticultural applications.     

Structure variation of tensile-deformed amorphous poly(l-lactic acid): Effects of deformation rate and strain

The present work explored the structure-property correlations for the biopolymer poly(l-lactic acid) (PLA) by studying deformation-mediated molecular orientation and crystallization. The structural and morphological variations of amorphous PLA under different strain rates were investigated. The result showed that strain rate significantly influences its strain-hardening behavior. The crystallinity and orientation as well as cavitation of deformed PLA increase with the increase of strain rates. The structure evolution has been divided into three potential stages: (i) at small strains (<100%), the crystallinity of PLA increases by orientation-induced crystallization; (ii) at intermediate strains (100%–160%), the crystallinity of deformed PLA slightly decreases due to the breakage of existing crystals under stress accompanying with newly formed voids and cavities; (iii) at high strains (>160%), the increasing number of oriented chains in the amorphous regions promotes the crystallization of PLA. Our study suggests that strain rate and stretching strain play important roles on modulating the crystallization and orientation of amorphous PLA.     

Synthesis and electrochromic properties of aromatic polyimides bearing pendent triphenylamine units

A series of aromatic polyimides with pendent triphenylamine group were synthesized from equimolar mixtures of 4,4′-oxydianiline (ODA) and 4-(3,5-diaminobenzamido)triphenylamine (4), 4-(3,5-diaminobenzamido)-4′,4″-di-tert-butyltriphenylamine (t-Bu-4) or 4-(3,5-diaminobenzamido)-4′,4″-dimethoxytriphenylamine (MeO-4) with two aromatic tetracarboxylic dianhydrides (DSDA or 6FDA) via a conventional two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s, followed by chemical imidization. These polyimides exhibited good solubility in polar organic solvents and could be solution-cast into flexible and strong films. They showed excellent thermal stability, with T_g values in the range of 284–309 °C. The polyimides derived from diamines t-Bu-4 and MeO-4 exhibited reversible electrochemical oxidation, accompanied by strong color changes with high contrast ratio and electrochromic stability. For the polyimides derived from diamine 4, the coupling reaction between the triphenylamine radical cations occurred during the oxidative process forming a tetraphenylbenzidine structure, which resulted in an additional oxidation state and color change together with enhanced near-IR absorption at fully oxidized state.     

Electrochemically and electrochromically stable polyimides bearing tert-butyl-blocked N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units

A new class of electrochemically active polyimides with di-tert-butyl-substituted N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units was prepared from N,N-bis(4-aminophenyl)-N′,N′-bis(4-tert-butylphenyl)-1,4-phenylenediamine and various aromatic tetracarboxylic dianhydrides via a conventional two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s, followed by chemical or thermal cyclodehydration. Most of the polyimides are readily soluble in many organic solvents and can be solution-cast into tough and amorphous films. They had useful levels of thermal stability, with relatively high glass-transition temperatures (276-334 °C), 10% weight-loss temperatures in excess of 500 °C, and char yields at 800 °C in nitrogen higher than 60%. Cyclic voltammograms of the polyimide films cast on the indium-tin oxide (ITO)-coated glass substrate exhibited two reversible oxidation redox couples at 0.70-0.74 V and 1.05-1.08 V vs. Ag/AgCl in acetonitrile solution. The polyimide films revealed excellent stability of electrochromic characteristics, with a color change from colorless or pale yellowish neutral form to green and blue oxidized forms at applied potentials ranging from 0.0 to 1.3 V. These anodically coloring polymeric materials exhibited high optical contrast of percentage transmittance change (Δ%T) up to 44% at 413 nm and 43% at 890 nm for the green coloration, and 98% at 681 nm for the blue coloration. After over 50 cyclic switches, the polymer films still exhibited good redox and electrochromic stability.     

The effects of spacer arms in cross-linked hyaluronan hydrogel on Fbg and HSA adsorption and conformation

Four hyaluronan based hydrogels with different cross-linking agents were synthesised. Compounds with an increasing hydrophobic character and length as spacer arms were chosen, i.e. O,O′-bis(2 aminopropyl)polyethylen glicol, 1.3 diaminopropane, 1.6 diaminohexane, and 1.12 diaminedodecane. The cross-linking reaction involved 50% of the carboxylate groups present in the hyaluronan macromolecule chain. Hydrogels were characterised by water-uptake measurements and SEM analysis. The adsorption of two plasma proteins (HSA and Fbg) was analysed onto the four hydrogels in flow conditions by ATR-FTIR, evaluating the adsorption kinetics and the eventual protein conformational changes. The increasing hydrophobic character of the surface imposes a clear trend to the protein adsorption kinetics. The influence of the spacer arms on the protein conformation is evident on HSA, whereas Fbg does not seem to be significantly influenced by the change of the substrate hydrophobic properties.     

Crystallization, structure and properties of plasticized poly(l-lactide)

Poly(l-lactide) (PLA) was plasticized with poly(ethylene glycol)s having M_w of 400 and 600 g/mol. In addition to poly(ethyne glycol)s with hydroxyl end groups, monomethyl ethers of poly(ethylene glycol) having M_w of 550 and 750 g/mol, with chains terminated with hydroxyl groups and methyl groups, were used. The effect of different end groups on the plasticization of both amorphous and semicrystalline PLA was studied. The crystallization, structure, thermal and tensile properties of PLA and PLA with 5 and 10 wt% of plasticizers were explored. No marked effect induced by different end groups of plasticizers was found. All the plasticizers used decreased T_g and increased the ability of PLA to cold crystallization. While an amorphous plasticized PLA could be deformed to about 550%, a semicrystalline PLA with the same total plasticizer content exhibited nonuniform plasticization of the amorphous phase and less ability to the plastic deformation. Nevertheless, a 20% elongation at break was achieved for a semicrystalline PLA with 10 wt% of the plasticizer. The plastic deformation of both neat and plasticized PLA was associated with crazing.     

Opto-thermal properties of fibers. XI. The effect of thermal annealing of drawing behavior for Egyptian polyester fibers on the crystallinity and some optical parameters

The structure developed of annealed Egyptain poly(ethyleneterephthalate) (PET) fibers is studied interferometerically due to the drawing process. Using a two-beam Pluta polarizing interference microscope connected to a device to dynamically study the draw ratio with the birefringence changes, the relations of drawing changes with some optical parameters are given. The evaluation of density, the mean square density fluctuation η², crystallinity, amorphous orientation, crystalline orientation functions, number of chains per unit volume N_c, and number of random links between the network junction points N has been found. The results obtained clarify the effect of annealing time and temperature with different draw ratios on the structure of PET fibers. Empirical formula is suggested to correlate the changes in f_θ, θ, Δn_a, and A with the draw ratio. Microinterferograms and curves are given for illustration. © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 68: 1371–1386, 1998     

Ultradrawing and crystallization of isotactic polystyrene and blends with atactic polystyrene

Pure isotactic polystyrene (iPS, M_w = 8.89 × 10⁴, M_w/M_n = 4.89) and its blends with an atactic polystyrene (aPS, M_w = 3.9 × 10⁵, M_w/M_n < 1.13) were subjected to draw by solid state coextrusion at 127°C within polyethylene. The content of amorphous iPS in these blends was varied from 100 to 24.4 wt %. The extent of draw-induced crystallization was found to depend on the draw ratio and on iPS concentration. The blend with 24.4% iPS was coextruded in two stages. The highest effective draw ratio (EDR) was 7.6 and 13.7 for one- and two-stage draw, respectively. The highest crystallinity of 33.2% was obtained for pure iPS at the maximum EDR of 7.6. Considerable crystallinity was induced in blends, requiring successively higher draw ratio to reach similar crystallinity with increased aPS content. The tensile modulus increased from 1.5 to 3.2 GPa, independent of iPS concentration. Thermal shrinkage results indicate that the elastic recovery of draw in the blends is near quantitative for an EDR < 8. For pure iPS, extrudate elastic recovery was dramatically altered by the draw-induced crystallinity.     

Structure development of poly(L‐lactic acid) fibers processed at various spinning conditions

Poly(L ‐lactic acid) (PLA) filaments were spun by melt‐spinning at 500 and 1850 mm^?1, and further drawn and heat‐set to modify the morphology of these PLA filaments. PLA yarns were characterized by wide‐angle X‐ray diffraction (WAXD) and sonic method. WAXD reveals that PLA yarns spun at 500 mm^?1 are almost amorphous while the PLA filaments spun at 1850 mm^?1 have about 6% crystallinity. This is different from PET filaments spun at the same speed that have almost no crystallinity. Both drawn‐ and heat‐set PLA filaments showed much higher crystallinity (60%) than do as‐spun fibers produced at 500 and 1850 mm^?1 speed, which is also higher than the usual heat‐set PET yarns. It appears that crystalline orientation rapidly reaches a value in the order of 0.95 at 1850 mm^?1 and that drawn‐ and heat‐set yarns have almost the same crystalline orientation values. Molecular orientation is relatively low for as‐spun PLA yarn, and molecular orientation increased to ～0.5 after drawing or heat–setting or both. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1210–1216, 2006     

Modeling of density evolution of PLA under ultra-high pressure/temperature histories

The density evolution of a semi-crystalline Poly(lactic acid) under ultra/high pressure histories in the region of alpha relaxation has been studied. It was found that the degree of crystallinity does not change in the temperature and pressure ranges under concern and consequently it is assumed that the volume relaxation kinetics involves only the amorphous part of the polymer. The scaling law for relaxation times, recently proposed by Casalini and Roland, has been utilized in the framework of KAHR (Kovacs Aklonis Hutchinson and Ramos) phenomenological theory in order to model the volume relaxation behavior. With this approach the model contains only two fitting parameters, (namely the relaxation time in the reference state, τ_g, and the fractional exponent, β, that describes the dispersion of the alpha relaxation) instead of the five parameters of the classical KAHR theory. The model predicts accurately both the density evolution of the PLA during the sterilization and Pasteurization treatments simulated in a pressure dilatometer and the density changes occurring in the PLA films utilized as packaging material in the HP industrial processes.     

Synthesis and characterization of highly functionalized polymers based on N,N,N′,N′-tetraalkyl-4,4′-diaminostilbene and maleic anhydride

Novel, highly functionalized rod-like copolymers have been synthesized by alternating copolymerization of N,N,N′,N′-tetraalkyl-4,4′-diaminostilbenes (TDASs) with maleic anhydride. These unique copolymers have been characterized by SEC, DSC and TGA. The solubility of these copolymers in organic solvents is strongly dependent on the length of the alkyl chains on the amino groups and the solubility in aqueous media is pH dependent. Light scattering studies indicate that the conformation of the polymer backbone does not change upon increasing temperature or introducing charges to the amino groups. High chain rigidity is further corroborated by the high T_g of the polymers. There is no observed glass transition below 280 °C. The light scattering and thermal results are indicative of a rod-like backbone structure.     

Copolyimides from 2,3,3′,4′-biphenyltetracarboxylic dianhydride and pyromellitic dianhydride with 4,4′-oxydianiline

A series of copolyimides were prepared via the polyamide acids (polyamic acids) from the reaction of 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA) and pyromellitic dianhydride (PMDA) with 4,4′-oxydianiline (4,4′-ODA) at dianhydride molar ratios of 9:1, 7:3, 1:1, 3:7 and 1:9. Homopolymers and a 1:1 polymer blend were also prepared. Films from the 7:3, 1:1 and 3:7 molar ratio polyamide acids reacted for 5-6 h at ambient temperature were brittle, whereas films from the same polyamide acids reacted for 24-48 h at ambient temperature were fingernail creaseable. The difference was apparently due to the initial formation of incompatible block domains that underway randomization upon longer reaction time. The differential scanning calorimetric (DSC) curves of some of the brittle films quenched after heating to 400 °C had two apparent glass transition temperatures (T_gs), indicative of two block domains. The creaseable films quenched after heating to 400 °C had single T_gs. Wide-angle X-ray diffraction showed all films to be amorphous even though the initial DSC curves showed strong endothermic peaks, generally associated with crystalline melts. These strong endotherms near the T_g region were thought to be due to relaxation of regions in the highly stressed films. Films of copolyamide acids from the reaction of 1:1 molar ratios of 3,3′,4,4′-oxydiphthalic anhydride/a-BPDA and 3,3′,4,4′-biphenyltetracarboxylic dianhydride/a-BPDA with 4,4′-ODA reacted for 6 h were fingernail creaseable. The chemistry and the properties of the copolymers are compared with those of the homopolymers.