Preparation and Characterization of Poly(L-lactic acid)/Polybutadiene Block Copolymers

A two-step process to prepare poly(L-lactic acid) (PLLA)/polybutadiene (PB) block copolymers was studied. Melt polymerization was used to prepare poly(L-lactic acid) terminated with hydroxy groups, at the same time hydroxy-terminated polybutadiene and 1,6-hexamethylene diisocyanate were employed to synthesize isocyanate-terminated polybutadiene (ITPB). Then PLLA and ITPB were reacted with different PB/PLLA weight ratios. Consequently, a series of PLLA/PB copolymers with linear or crosslinked chains was obtained. Viscosity of the linear copolymer was measured by an Ubbelohde viscometer. Swelling characteristics and crosslink density of the crosslink copolymer were investigated. Fourier transform infrared and proton nuclear magnetic resonance were used to characterize the structure of the copolymer, and dynamic mechanical analysis (DMA) was applied to characterize its thermal properties. Mechanical property measurements showed that a toughened PLLA polymer was synthesized. Atomic force microscope was utilized to characterize its micro-morphology.     

Designing and Characterization of Poly(L-Lactide)/Poly(ϵ-Caprolactone) Multiblock Copolymers

A series of poly(L-lactide)/poly(?-caprolactone) (PLA/PCL) biodegradable multiblock copolymers was synthesized by a two-step process and characterized. Ring-opening polymerization was used to prepare a series of HO-PLA-PCL-PLA-OH copolymers initiated by hydroxyl-terminated PCL. Then the triblock copolymers and 1,6-hexamethylene diisocyanate (HDI) were reacted with different copolymer/HDI weight ratios. Consequently, a series of PLA/PCL multiblock copolymers with designed molecular chain structure was obtained. Gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, and ¹H NMR were used to characterize these copolymers and the results showed that the designed PLA/PCL copolymers had been synthesized. Dynamic mechanical analysis (DMA) was applied to characterize their thermal properties. Stress–strain curves showed that a PLA/PCL copolymer with adjustable mechanical properties had been achieved.     

Properties of acrylonitrile butadiene rubber (NBR)/poly(lactic acid) (PLA) blends and their foams

Abstract

Thermoplastic elastomers and their foams were prepared by blending elastomeric acrylonitrile butadiene rubber (NBR) and rigid poly(lactic acid) (PLA) with various PLA compositions ranging between 0 and 40%. The thermal and mechanical properties and the morphologies of the blends with various PLA contents were investigated through universal testing machine, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscope analysis. The rheological properties during gel formation were in situ monitored through the evolution of torque with curing time. Furthermore, the microcellular structures and physical properties of the NBR/PLA foams prepared using organic blowing agents were studied. The NBR/PLA blends showed a two-phase morphology made of a continuous NBR matrix and micron or submicron nodules and the tensile strength and modulus; also, hardness of the NBR/PLA blends increased with the increase of the added PLA content. While the foamed samples exhibited a similar cell structure and foaming ratio to that of the pure NBR, the cell formation was considerably reduced as the added PLA content exceeded 30%. We conclude that the mechanical properties of NBR thermoplastic elastomer as well as its foams can be controlled by a judicious introduction of rigid and biodegradable PLA.     

Effect of chemical modifications of the pineapple leaf fiber surfaces on the interfacial and mechanical properties of laminated biocomposites

Natural fiber reinforced renewable resource based laminated composites were prepared from biodegradable poly(lactic acid) (PLA) and untreated or surface-treated pineapple leaf fibers (PALF) by compression molding using the film stacking method. The objective of this study was to determine the effects of surface treatment of PALF on the performance of the fiber-reinforced composites. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to aid in the analysis. The mechanical properties of the PLA laminated composites were improved significantly after chemical treatment. It was found that both silane- and alkali-treated fiber reinforced composites offered superior mechanical properties compared to untreated fiber reinforced composites. The effects of temperature on the viscoelastic properties of composites were studied by dynamic mechanical analysis (DMA). From the DMA results, incorporation of the PALF fibers resulted in a considerable increase of the storage modulus (stiffness) values. The heat defection temperature (HDT) of the PALF fiber reinforced PLA laminated composites was significantly higher than the HDT of the neat PLA resin. The differential scanning calorimeter (DSC) results suggest that surface treatment of PALF affects the crystallization properties of the PLA matrix. Additionally, scanning electron microscopy (SEM) was used to investigate the distribution of PLA within the fiber network. SEM photographs of fiber surface and fracture surfaces of composites clearly indicated the extent of fiber–matrix interface adhesion. It was found that the interfacial properties between the reinforcing PALF fibers and the surrounding matrix of the laminated composite are very important to the performance of the composite materials and PALF fibers are good candidates for the reinforcement fiber of high performance laminated biodegradable biocomposites.     

Effect of elastomer matrix type on electromechanical response of conductive polypyrrole/elastomer blends

Electrorheological properties of prestine elastomers and polypyrrole/elastomer blends were investigated to identify the most suitable elastomer to be used in electroactive actuator applications. Seven types of elastomer: poly(acrylate) copolymers (AR70, AR71, AR72, and SAR), poly(dimethyl siloxane) (PDMS), poly (styrene butadiene) (SBR), and poly(styrene isoprene styrene) (SIS) were chosen as the candidate dielectric elastomers. For the pure elastomers, the storage modulus responses, ΔG′, amongst the elastomers differ by 6 orders of magnitude at the electric field strength of 2 kV/mm. The storage modulus sensitivity, ΔG′/G′₀, increases with electric field strength and attains maximum values of 97% for SBR, 148% for SAR, 232% for AR70, 13% for PDMS, 69% for AR71, 54% for AR72, and 10% for SIS at the electric field strength of 2 kV/mm. Correlations are identified between the elastomer storage modulus sensitivity and G′₀ and/or electrical conductivity. For the undoped polypyrrole/elastomer blends with the particle concentrations of 1, 2, 3, 4, and 5 vol%, ΔG′ increases linearly with concentration in the absence of electric field but nonlinearly with electric field on, due to the obstruction of the matrix-dipole interaction at small concentrations and the nonuniform polypyrrole particle dispersion in the matrix at high concentrations.     

Toughening poly(lactic acid) (PLA) through in situ reactive blending with liquid polybutadiene rubber (LPB)

Abstract

Liquid polybutadiene rubber (LPB) was blended with poly(lactic acid) (PLA) through reactive and non-reactive routes to enhance the toughness of the PLA. The reactively blended PLA (PBR10) was prepared by melt blending the PLA with the LPB in the presence of dicumyl peroxide (DCP), a radical initiator, while the PB10 was just melt blended without the DCP. Fourier transform infrared (FTIR) spectra and wide-angle X-ray diffraction (WAXD) patterns were used to study the molecular structure of the blends. Properties were investigated through universal testing machine (UTM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscope (SEM) analysis, and rheological measurements. The results indicated that the radical crosslinking by the DCP could increase the compatibility between the PLA and LPB and disperse the rubber particles at the nanoscale in the PLA matrix. As a result, the toughness and melt viscosity of the PLA was significantly enhanced through the reactive blending, which is promising for the practical application of the modified PLA in the area of packaging.     

Effect of surface modified hydroxyapatite on the tensile property improvement of HA/PLA composite

In this study, we modified the surface of hydroxyapatite (HA) particle by ring-opening polymerization of lactide (LA). The modified HA particles were characterized by IR and TGA. It was shown that LA could be graft-polymerized onto the surface of HA. A series of composites based on modified HA/PLA were further prepared and characterized. It indicated that the modified HA particles were well dispersed in PLA matrix than unmodified HA particles and the adhesion between HA particle and PLA matrix was improved. The modified HA/PLA composites showed good mechanical properties than that of unmodified HA/PLA.     

(Stress at Break)/(Shore A) Balance in Propylene/Ethylene Elastomers

Propylene/ethylene elastomers with ethylene content in the range 10–20 wt % are known to exhibit elastomeric properties when ethylene is randomly distributed along the polypropylene chains: this condition favors the formation of small crystallites during solidification from the melt and mechanical straining. These small crystals act as physical crosslinks for the rest of the amorphous chains and promote elastomeric behavior. Based upon these concepts a mathematical relationship between stress-at-break and Shore A hardness, was developed in the form of an S-shaped curve and validated with a series of polymers containing different amounts of ethylene. These polymers were produced with both Ziegler-Natta (at 100°C) and metallocene catalysis. It was seen that the catalysis type has a marginal influence on elastomer performances. There is a slightly better balance between Shore A and stress-at-break at low ethylene contents for the metallocene elastomers whereas the Ziegler-Natta catalysis results in elastomers with better thermal properties. Addition of PP homopolymer worsens the balance between Shore A and stress-at-break; the same happens following the addition of inorganic filler. In contrast, the addition of a low-molecular-weight paraffinic hydrocarbon promotes softer elastomers with the same stress-at-break/Shore A balances of the parent elastomers.     

Modelling of the viscoelastic behavior of homogeneous and heterogeneous blends of polyisoprene and polybutadiene

The mechanical model of Takayanagi has been used to describe the viscoelastic behavior of various blends of a ternary elastomer system (cis-1, 4 polyisoprene, 1, 4-polybutadiene, and the corresponding polyisoprene/polybutadiene diblock) for which extensive mechanical properties data have already been reported. Through the use of appropriate composition-temperature shifts which rely on the assumption of free volume additivity, it was possible to model the behavior of the homogeneous diblock copolymers of polyisoprene and polybutadiene as well as various homogeneous binary and ternary blends of a diblock with either or both of the diene homopolymers. Heterogeneous ternary blends were also modelled successfully by assuming that the diblock was solubilized preferentially by one of the phases in the blend. The model predictions were very sensitive to the location of the diblock, and thus comparison with experimental results provided a useful tool for verifying earlier assumptions regarding the role of the homogeneous diblock copolymers in these elastomer blends.     

Activation Energy for the Glass Transition of a Confined Elastomer in HDPE/PP Blends

Blends of two highly crystalline polymers containing an elastomer were prepared to study the glass transition of the confined elastomer. The polymers chosen were high density poly ethylene (HDPE), polypropylene (PP), and two elastomers of a different nature: natural number (NR) and EPDM. The dynamic mechanical analyzer (DMA) technique was used to analyze the storage modulus of blends with elastomer content from 0% to 30% by weight, with the remainder made up of equal amounts of HDPE and PP, and blends with 10% of the elastomer, but varied ratios of polyolefins. We used the differentiation modification of the Arrhenius method in the kinetic analysis assuming an n‐order relaxation mechanism, which allowed detecting the percolation threshold of NR. Results indicate that both temperature and activation energy for glass transition (T _g ) are dependent on the types of polymers in the blend and blend composition. The T _g and E values of the unblended elastomers are higher than those in blends; this behavior is associated with the elastomer confinement and blend morphology.     

Correlation of the Morphological and Mechanical Properties of a Biodegradable Blend Based on Polylactic Acid

A series of binary and ternary blends composed of polylactic acid (PLA), low-density polyethylene (LDPE), and chitosan (CS) were prepared and characterized in terms of their morphological and mechanical properties. The mechanical properties of the prepared blends, including tensile properties and impact strength, were compared with neat PLA. In addition, the effect of incorporation of maleic anhydride-grafted linear low-density polyethylene (LLDPE-g-MA) as a compatibilizing agent, and the order of mixing on the mechanical and morphological properties of the ternary blends were also studied. It was observed that addition of CS enhanced the stiffness of PLA/LDPE blends while it decreased the toughness and tensile strength. It was demonstrated that addition of LLDPE-g-MA, up to 10 wt%, had no significant compatibilizing effect. However, the mechanical results indicated that when 15 wt% of LLDPE-g-MA was loaded, it started to play a compatibilizing role and caused an improvement in the toughness properties of ternary blend.     

Nonisothermal Crystallization Kinetics of Poly(lactic acid) Nucleated with a Multiamide Nucleating Agent

Addition of a commercial available multiamide compound (N,N′,N′′-tricyclohexyl-1,3,5- benzenetricarboxylamide, defined here as TMC) into ecofriendly poly(lactic acid) (PLA) can accelerate the crystallization rate of the material remarkably and broaden its applications. In this paper, the nonisothermal crystallization behavior of biodegradable PLA nucleated by 0.3 wt.% of TMC was investigated by differential scanning calorimetry (DSC). The modified Avrami, Tobin, Ozawa, and Mo models were applied to describe the kinetics of the crystallization process. Various parameters of nonisothermal crystallization, such as the crystallization half-time and crystallization rate constant, reflected that TMC significantly accelerated the crystallization process. The activation energy values of the neat PLA and PLA/TMC blend, determined by the Kissinger method, increased with the addition of TMC. The study should be helpful for understanding the relationship between processing and properties of this material.     

Preparation and characterization of poly(lactic acid)/poly(methyl methacrylate) blend tablets for application in quantitative analysis by micro Raman spectroscopy

Poly(lactic acid) (PLA) is a biodegradable polymer that has a variety of applications, one of which is as biomaterial in surgery or as functional layers on implants, due to its compatibility with living tissue. This paper reports the possibilities of quantification of poly(lactic acid) (PLA) in a polymer matrix such as poly(methyl methacrylate) (PMMA) by micro Raman spectroscopy (MRS). Blends of amorphous poly(DL‐lactic acid) with poly(methyl methacrylate) were prepared by the procedure of dissolution/precipitation. Thermal properties of the blends such as the glass transition temperature (Tg) were characterized by differential scanning calorimetry (DSC). The PLA/PMMA blends exhibited only a single glass transition region, indicating that this system is miscible. The PLA/PMMA system obeys the Gordon–Taylor equation (Tg versus PLA content). Various concentration ratios of PLA blends were prepared to use as a basis for quantitative analysis by MRS. Intensities of the characteristic bands at 813 cm⁻¹ (νCOC of PMMA) and 873 cm⁻¹ (νC―COO of PLA) were used for the calculation. The calibration graph showed a good linear correlation with an R² value of 0.9985. On the basis of the calibration curve obtained, the determined content of several PLA/PMMA blends was in good agreement when compared with nominal contents. The limit of detection (LOD) and quantification (LOQ) were calculated by the calibration data set as signal‐to‐noise method. The relative standard deviation of this method was lower than 10% and the accuracy better than 4%. This study demonstrated that Raman spectroscopy provides an alternative non destructive method for quantitative analysis of PLA in a PMMA matrix. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and Characterization of Biodegradable Poly(lactic-co-glycolic acid)

A series of poly(lactic-co-glycolic acid) (PLGA) was synthesized by bulk ring-opening copolymerization with different ratios of L-lactic acid (L-LA) to glycolic acid (GA); stannous octoate [Sn(Oct)2] was used as catalyst. The structures were characterized by FT-IR. The crystallinity and the glass-transition temperature (T_g) were studied by differential scanning calorimeter (DSC). T_g decreased with the increase of GA and the rate of degradation and degree of hydrophilicity increased with the increased content of GA.     

Surface investigation of adhesive formulation consisting of UV sensitive triblock poly(styrene–b-butadiene–b-styrene) copolymer

Atomic force microscopy (AFM) analysis in conjunction with macroscopic studies such as peel testing and contact angle measurement have been undertaken to explain the nanomechanical properties of adhesive formulation consisting of triblock poly(styrene–b-butadiene–b-styrene) (SBS) copolymers. The cross-linking of this photosensitive copolymer was investigated by analyzing the mechanical and morphological changes of each phase induced by the UV exposure. Main result is that the adhesive properties are strongly influenced by the cross-linking of the polybutadiene (PB) phase leading to an increase in the surface stiffness without affecting the surface energy. AFM analysis shows that the adhesion force is mostly governed by the contact area between the adhesive and the probe. The surface mobility may explain the increase in adhesion for this pressure sensitive copolymer.     

The Effect of Poly(Ethylene Succinate) on Mechanical Properties of PLLA/PES Blend Prepared by Melt-Blending

Fully biodegradable poly(L-lactide) and poly(ethylene succinate) (PLLA/PES) blends were prepared via melt-blending using PLLA and PES as reactants in a stainless steel chamber. The prepared PLLA/PES blend, as well as neat PLLA and PES, was characterized by Fourier transform infrared spectra (FTIR) and X-ray diffraction (XRD) to confirm the structure and the crystallization of PLLA in the blend. The mechanical properties of PLLA/PES blends were determined by bending and tensile tests and the effects of PES content on the mechanical properties of PLLA/PES blends were investigated. It was found that blending some amount of PES could significantly improve the elongation at break while still keeping considerably high strength and modulus. With increasing PES content, both strength and modulus gradually decreased; however the elongation at break significantly increased. SEM was used to examine the morphology of fracture surfaces of PLLA/PES blends.     

Morphology and Physical Properties of ABS/NBR: The Effect of Melt Viscosity of SAN and the Content of NBR

In several acrylonitrile-butadiene-styrene (ABS) copolymers, some amounts of polybutadiene (PB) laTeX grafted with styrene-acrylonitrile (SAN) copolymer were replaced by acrylonitrile-butadiene rubber (NBR) copolymer, and the variations of morphology, mechanical properties, and rheological properties were examined. The impact strength of ABS, with a bimodal distribution of rubber size, was improved by the presence of the NBR, which distributes coarsely in the SAN matrix. Yield behavior in the rheological response due to the presence of rubber particles in the SAN matrix was enhanced by the coarser NBR particles, especially at high temperature.     

Thermal and Thermo-Oxidative Degradation of Biodegradable Poly(Ester Urethane) Containing Poly(L-Lactic Acid) and Poly(Butylene Succinate) Blocks

A novel biodegradable poly(ester urethane; PEU) was synthesized by chain extension reaction of dihydroxylated poly(L-lactic acid; PLLA) and poly(butylene succinate; PBS) using diisocyanate as a chain extender. The kinetics of thermal and thermo-oxidative degradation of PEU containing PLLA and PBS blocks were studied by thermogravimetric analysis (TGA). TGA results indicated that PEU was more stable in air than in nitrogen and went through a two-stage degradation process irrespective of the experimental atmosphere. Activation energy of each stage was calculated by means of Kissinger, Kim-Park, Friedman, Flynn-Wall-Ozawa, and Kissinger-Akahira-Sunose methods. For the first stage, the activation energy value obtained in air was slightly higher than the corresponding value obtained in nitrogen; and for the second stage, the activation energy showed a much higher value in air than in nitrogen. The Coats-Redfern method was employed to study the degradation mechanism of each stage. The results indicated that the degradation of the first stage follows the P3/4 mechanism irrespective of the experimental atmosphere; the degradation of the second stage of PEU obeys the P1 mechanism in nitrogen while P3/2 in air.     

Comparison of Structure,Morphology, and Properties of Miktoarms Star Styrene-Butadiene Rubber and Star-Shaped Styrene-Butadiene Rubbe/Polybutadiene Rubber Blends

Four miktoarms star-shaped polybutadiene-Sn-poly(styrene-butadiene) rubber (MSS-PB-PSBR) with 1,1-diphenylhexyl at the ends of the arms were prepared by two different coupling techniques. One technique was a one-step technology, from which two miktoarms star styrene-butadiene rubbers, called AMSS-PB-PSBR, were obtained in which the four arm stars had varying ratios of PB:PSBR arms; another was a two-step technology, from which another two miktoarms star styrene-butadiene rubbers, called BMSS-PB-PSBR, were obtained in which all consisted of PB-Sn-(PSBR)₃ stars. The molecular structure parameters and morphology-properties of the four MSS-PB-PSBR were determined and studied, and compared with that of a star-shaped styrene-butadiene rubber (S-SSBR)/poly butadiene rubber (PBR) blend. The results showed that the total coupling efficiency (the ratio of the total number of polymer chains (arms) coupled by SnCl₄ to that of the total number of polymer chains) of the MSS-PB-PSBR was higher than 60%. However, the coupling efficiency of the polybutadiene arms of BMSS-PB-PSBR was obviously higher than that of the AMSS-PB-PSBR. Compared with the S-SSBR/PBR blend, MSS-PB-PSBR had a more uniform distribution of the PB phase and a smaller phase size of PB. It was found that MSS-PB-PSBR composites filled with carbon black (CB) had a lower Payne effect than the S-SSBR/PBR/CB composite, with the BMSS-PB-PSBR/CB composites being especially lower. The BMSS-PB-PSBR/CB composites had higher mechanical properties and lower rolling resistance than the AMSS-PB-PSBR/CB composites due to the high coupling efficiency of the polybutadiene arms; the results indicated that the two-step technology was better than the one-step technology for preparing the tread material of “green” tires.     

Thermal,Mechanical, and Dielectric Properties of a Dielectric Elastomer for Actuator Applications

Dielectric elastomers (DE) are a new type of electro-active material, which is able to produce a large degree of deformation under electrical stimulation. The thermal, mechanical, and dielectric properties of the most widely used dielectric acrylic elastomer (VHB 4910), commercially available from the company 3M, were studied by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and broadband dielectric spectroscopy (BDS) analyzer, respectively. DSC experiments on the VHB 4910 showed a glass transition at about ?40°C. VHB 4910 started to lose weight at about 250°C from the TGA study. The results of DMA indicated the storage modulus of VHB 4910 increased with frequency and had a strong temperature dependence of elasticity. The dielectric constant of VHB 4910 increased as a function of temperature up to 0°C, followed by a drop till 100°C. The mechanical and electrical efficiency of dielectric elastomer actuators (DEA) of VHB 4910 were analyzed. It was demonstrated that the actuation performance is dominated by the mechanical properties of the elastomer and is less influenced by the frequency and the temperature dependence of the dielectric properties; this may be used to guide the design of actuator configurations, as well as the choice of actuator materials.