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A composite of 70/30 poly(lactic acid)/hydroxyapatite was systematically prepared using various amounts of glycidyl methacrylate as reactive compatibilizer or Joncryl ADR®-4368 containing nine glycidyl methacrylate functions as a chain extension/branching agent to improve the mechanical and biological properties for suitable usage as internal bone fixation devices. The effect of glycidyl methacrylate/Joncryl on mechanical properties of poly(lactic acid)/hydroxyapatite was investigated through flexural strength. Cell proliferation and differentiation of osteoblast-like MC3T3-E1 cells cultured on the composite samples were determined by Alamar Blue assay and alkaline phosphatase expression, respectively. Result shows that flexural strength tends to decrease, as glycidyl methacrylate content increases except for 1 wt.% glycidyl methacrylate. With an addition of dicumyl peroxide, the flexural strength shows an improvement than that of without dicumyl peroxide probably due to the chemical bonding of the hydroxyapatite and poly(lactic acid) as revealed by FTIR and NMR, whereas the composite with 5 wt.% Joncryl shows the best result, as the flexural strength increases getting close to pure poly(lactic acid). The significant morphology change could be seen in composite with Joncryl where the uniform agglomeration of hydroxyapatite particles oriented in poly(lactic acid) matrix. Addition of the epoxy functional compatibilizers at suitable percentages could also have benefits to cellular attachment, proliferation, differentiation and mineralization. So that, this poly(lactic acid)/hydroxyapatite composite could be a promising material to be used as internal bone fixation devices such as screws, pins and plates.

Curable precursors for production of thermosetting (co)polyesters are developed based on poly(butylene adipate) (PBA), poly(ethylene terephthalate), and poly(lactic acid) (PLA). The precursors are prepared from methacrylation of their hydroxyl-terminated oligomers. These include commercial HO-capped PBA prepolymer (HO-PBA), and bis-2-hydroxyethyl terephthalate (BHET) and glycolized PLA (GPLA), which are obtained from glycolysis reactions of their post-consumer products and original resin. The optimal conditions for methacrylation of each prepolymer, i.e., molar ratios of prepolymer to methacrylic anhydride (MAAH), temperature and time, are examined. The most efficient conditions are 1:4/120 °C/3 h (HO-PBA), 1:2.5/120 °C/2 h (BHET), and 1:4/140 °C/3 h (GPLA), respectively. These conditions are justified, where the highest degree of substitution and double bond content are obtained without self-curing side reaction. The resulting methacrylated precursors from HO-PBA and BHET are isothermally cured faster than those of GPLA. Thermosetting copolyesters developed from binary mixtures of the precursors possess tunable thermal stability, and physical and mechanical properties. The degradable copolyesters can be applied in various applications, especially in packaging and agricultural fields.

Bio-degradable/bio-compatible poly(lactide-co-glycidyl methacrylate), P(LA-co-GMA), a copolymer has been synthesized. The material contains curable C C groups, which enable its self-curing and grafting reactions with other vinyl monomers. The copolymer was grafted with a pH-responsive polyacrylamide (PAAm), by UV-assisted reactions using acrylamide (AAm) and N,N′-methylene bisacrylamide monomers, and various photoinitiator systems. The original copolymer and its partially-cured counterpart were employed in the grafting reaction. Chemical structures and properties of the resulting materials were characterized. Standard quantitative analysis techniques for measurement of the grafted AAm content and the degree of C C conversion have been developed by 1H NMR and FTIR spectroscopy. FTIR offers more advantages, in terms of non-destructive analysis, ease of operation, and lower cost of analysis. The results show that the grafted products from pre-cured P(LA-co-GMA) copolymers contain higher grafted AAm contents than their uncured counterparts. The highest grafted AAm content was obtained by using benzophenone (BP) as an initiator, while camphorquinone (CQ) led to the lowest content. In contrast, the degree of C C conversion of the copolymer from the two initiator systems shows a reverse trend. These amphiphilic and pH-responsive grafted copolymers with tunable AAm contents have a high potential for use in various applications, especially in biomedical and environmental fields.

A process for the chemical recycling of degradable polylactide (PLA) has been developed, to convert its disposable wastes into small-sized products for further use as value-added starting materials for specialty products. A microwave-assisted alcoholysis reaction, using diols and tetrabutyl orthotitanate (TBT) catalyst, was employed. The effects of the types of diols and PLA/diol feeding ratios on the structures and properties of the resulting alcoholized products were examined by using ethylene glycol (EG), propane-1,3-diol (PDO), and butane-1,4-diol (BDO). The PLA/diol ratios were varied from 1:1 to 4:1 wt/wt. The products were characterized by ATR-FTIR, 1H NMR, and 2D-NMR HMQC and HMBC spectroscopy. The products compost to diol-capped lactate sequences with different lengths, i.e., lactate, lactyllactate, and poly-lactate. The yield of the shortest lactate was about 65% for the LBDOT11 system, and 90% for the LEGT11 and LPDOT11 systems, when a feeding ratio of 1:1 wt/wt was employed. The yield decreased with an increase in the ratios, from 1:1 to 4:1 wt/wt. In contrast, the yields of longer lactyllactate and poly-lactate increased with an increase in the ratios. In terms of OH-endcapping efficiency, the products have two structures: (A) LA-O-(CH2)x-OH and (B) LA-O-(CH2)x-O-LA. The contents depend on diol types and the PLA/diol feeding ratio. The alcoholized products were then used as starting polyols in the preparation of lactide-based polyurethanes (PUs) by reacting with 1,6-diisocyanatohexane (HDI) and BDO chain extender. The resulting PUs exhibit promising mechanical and thermal properties. These PUs are suitable for use as either single-component materials, e.g., 3D-printing filaments and degradable adhesives, or as toughness-enhancing agents for brittle polymers, especially PLA.

Nanofibers of polylactide (PLA)/poly(vinylpyrrolidone) (PVP) blends, loaded with TiO2 nanoparticles, have been prepared by an electrospinning method. The electrospun fiber mats were characterized by ATR-FTIR, X-ray diffraction (XRD), SEM, EDX, and UV-visible spectroscopy to examine structures, functional groups, crystallinity, surface morphology, and UV absorptivity. It is clearly observed that TiO2 particles are embedded on the filaments. All PLA-based spun fibers are completely amorphous in nature. The surface morphology of those blended with PVP is smoother and more uniform than the corresponding samples without PVP. Neat PLA fibers show a UV absorption band at around 200 nm, whereas the fibers loaded with TiO2 nanoparticles show an additional absorption band covering the 200–380-nm region. Photo-degradation of the fiber samples are conducted in phosphate buffer solution (PBS) under UVA light. The results indicate that the PVP component dissolves into the PBS solution, and the PLA matrix degrades as a function of time. The fibers are then applied as a catalytic system for epoxidation of unsaturated sunflower oil (SFO), for use as additives or plasticizers for biopolymers, employing a performic acid oxidizing agent. The fibers, especially those containing PVP, can effectively enhance the epoxidation yield of oils with a slow rate of undesirable side reactions, which break ester bonds of triglycerides to generate free fatty acids.

Summary A method for preparation of silica particles from rice husk ash (RHA) is developed for use as fillers in an enhancement of gas permeability of polylactic acid (PLA) resin. Silica particles are produced in a one-step process by employing PLA-grafted-chitosan copolymer (PCT) as a polymeric surfactant. Particles with an average size of ca. 30 micron, which are smaller than those obtained from a conventional method, are obtained. The major advantage of this technique is its ease of operation, in which mechanical grinding is not required. The resulting particles are then used in preparation of PLA/silica composites by varying the silica contents from 0–2.0 wt %. The PCT copolymer is present as a coating layer on the particles surface, which promotes compatibility with the PLA matrix. Gas permeability (water vapor, carbon dioxide, and oxygen) and mechanical properties of the composite films are examined. Enhancements in gas permeability and selectivity are achieved, with slight drops in mechanical properties. The materials have high potential for use as degradable packaging films with tunable gas permeability.

Hydroxyl-capped polylactic acid (PLA) oligomers are prepared by glycolysis reaction of PLA with ethylene glycol (EG) and used as a macromolecular cross-linker for epoxidized natural rubber (ENR). The glycolyzed PLA (GPLA) with three different molecular weights (2000, 10,000, and 44,000 g mol−1) are prepared by varying the glycolysis conditions. Effects of GPLA chain lengths and GPLA/ENR feed ratios on cross-linking efficiency, chemical structures, and physical properties of the cured products are investigated. The cross-linking efficiency is examined using a moving die rheometer, solvent fractionation, thermogravimetric analysis, and Fourier transform infrared spectroscopy. Optimum GPLA/ENR compositions for the curing reaction range from 20 wt% to 33 wt%, depending on the GPLA chain length. Because of their biodegradability, biocompatibility, and the ability to tune up their structures and properties, the cured rubber materials have high potential for use in various biomedical applications.

Multi-branched poly(D-lactide)s (mbPDLAs) with various structures are synthesized via ring-opening polymer- ization by using polyglycidol (PG) macro-initiators. Their chemical structures and thermal properties are controlled by adjusting feed ratios of D-lactide (DLA) and PG. The materials are blended with commercial linear poly(L-lactide) (l-PLLA) to form a stereocomplex structure. Effects of mbPDLAs structures and l-PLLA/mbPDLA ratios on the blends' thermal, mechanical, and rheological properties are evaluated. Mechanical properties of the stereocomplex blends, espe- cially elongation at break and toughness, are dependent on the blend compositions, in which a 90:10 ratio exhibits the most desirable properties. The material also exhibits the lowest complex viscosity, which provides easy processing conditions. This is achieved by the incorporation of copolymers with multi-branched structures and an ability to form a much stronger stereocomplex structure.

Glycolyzed polylactic acid (GPLA)-cured epoxidized natural rubber (ENR) is developed for use as a toughening agent for PLA resin. GPLA is obtained from chemical recycling of PLA resin by a glycolysis reaction. GPLA-cured ENR is then prepared by the crosslinking reaction of ENR with GPLA in an internal mixer. Chemical structures of the cured products are characterized by solvent fractionation and thermogravimetric analysis (TGA). The cured ENR products are blended with PLA resin, by varying the cured ENR contents from 5 to 15% wt. Mechanical properties of the blends, and their toughening mechanisms are examined. The cured ENR materials has higher efficiency in improving toughness of PLA resin, compared to uncured ENR, likely due to their rubbery network nature and higher compatibility with the PLA matrix. The incorporation of 5% wt. GPLA-cured ENR also improves elongation at break with no adverse effect on tensile strength and modulus of PLA.

Nanofibers of poly(L-lactide-co-DL-Lactide) (PDLLAx) copolymers with DL-lactate (DLLA) contents of 0, 2.5, 7.5, and 50%, which exhibit strong structure/properties correlation, were fabricated by electrospinning. Effect of the copolymer structure and electrospinning conditions on morphology and properties of the fibers were examined by SEM, DSC, XRD, and tensile measurements. Bead-free fibers of PDLLAx prepared from a DMF/CHCl3 mixed solvent are roughly 10-times smaller in size (600–800 nm), with lower degree of surface porosity, compared to those of CHCl3. When CHCl3 is employed, an increase in size (2.4–5.5 μm) and surface porosity (0–45%) with relative humidity value is observed in crystallizable copolymers, whereas an amorphous copolymer shows a reverse trend. Thermal properties and chain arrangements of the electrospun fibers are critically affected by DLLA content of the copolymers and electrospinning conditions, as a result from interplay between intermolecular and intramolecular hydrogen bonding. Contents of crystalline domains and “physical crosslinks” generated from DL lactate segments are proposed as the origin of this phenomenon. Fiber mats of PDLLA with 50% DLLA content show a large improvement in all aspects of mechanical properties, which are suitable for various biomedical applications. POLYM. ENG. SCI., 54:472–480, 2014. © 2013 Society of Plastics Engineers

Curable precursors are prepared from chemical recycling of degradable polylactic acid (PLA) for development of aliphatic polyester thermoset materials. PLA resin (NatureWork 4042D) was de-polymerized via glycolysis under various conditions to produce PLA glycolysates (GlyPLAs), whose chain-ends mainly consist of hydroxyl groups with $$\bar{M}_{n}$$ ranging from 3,600 to 17,000 g/mol. Unsaturated double bonds (DB) were introduced into GlyPLA structures by end-capping with methacrylic anhydride to generate curable LA-precursors. The end-capping efficiency is strongly dependent on the molecular weight of GlyPLAs, where smaller-sized glycolysates produce LA-precursors with higher DB content. Curing behaviors of the precursors are thoroughly examined. DSC and FTIR results show that curing reactions at 140 °C are completed after 2 h for all samples. Results on gel fraction indicate that LA-precursor with $$\bar{M}_{n}$$ ~ 3,600 g/mol is the most effective candidate for producing network products with high crosslink density.

Crosslinkable poly(lactic acid-co-glycidyl methacrylate), P(LLA-co-GMA), copolymer was systematically synthesized by ring-opening polymerization. Synthesis parameters, i.e., catalyst type, comonomer ratio and polymerization time and temperature were studied. The incorporated GMA content strongly affected the physical and thermal properties of the obtained copolymers, which varied from semi-crystalline to completely amorphous polymers. Melting and glass transition temperatures, and molecular weights of the copolymers decreased as the GMA content in the copolymer increased. Curing behavior of the copolymers was assessed by employing thermo- and photo-crosslinking processes, and the crosslink density was evaluated via their gel content and solvent swelling ratio. A photo-crosslinking process was proven as a practical method in the curing of the copolymers, as the reaction was almost complete within 2 min, as indicated by a gel content of 96%. In the thermo-crosslinking process, at least a 15-min curing time was required at 120 °C. The cured products of copolymers with 19.2 mol% GMA showed the highest compressive stress at 25.5 MPa. Random copolymer of poly(lactic acid-co-glycidyl methacrylate), P(LLA-co-GMA) was systematically synthesized by ring-opening polymerization. Synthesis parameters, that is, catalyst type, comonomer ratio and polymerization time and temperature were studied. The incorporated GMA content strongly affected physical and thermal properties of the obtained copolymers, which varied from semi-crystalline to completely amorphous polymer. Curing behaviors of the copolymers were assessed by employing thermo- and photo-crosslinking processes, where the crosslink density was evaluated via their gel content and solvent swelling ratio.

A series of 0–3 composites of lead zirconate titanate (PZT)/ferroelectric odd–odd nylons was prepared with variations in the size of PZT powder and in the dipole density of ferroelectric odd–odd nylons. The as-formed PZT powder prepared by a spray-drying technique consists of spherical granules composed of large numbers of interconnected fine particles. Electrical properties of the composites as a function of the size of this particular PZT powder and the dipole density of ferroelectric odd–odd nylons were studied. With the PZT powder sintered at 1100°C, the relative permittivity, polarization, and piezoelectric strain coefficient (d33) of the PZT/nylon composites increased with increasing powder size of PZT. The electrical properties of the composites also increased as the dipole density of the polymer increased. The PZT/nylon 57 composite exhibited a high-relative permittivity and piezoelectric strain coefficient d33 due to an increase in its dipole density. The optimum relative permittivity and polarization of this composite were obtained by the PZT with an average granule size of 95 μm. The highest piezoelectric strain coefficient d33 of the 0–3 composite was found at 28 pC/N whereas the relative permittivity was approximately 73.

Lactic acid-based aliphatic/aromatic copolyesters are synthesized to incorporate the degradability of polylactic acid and good mechanical properties of aromatic species by using polycondensation of lactic acid (LA), dimethyl terephtha- late (DMT), and various diols. Effects of diol lengths and comonomer feed ratios on structure and properties of the result- ing copolymers are investigated. Three types of diols with different methylene lengths are employed, i.e., ethylene glycol (EG), 1,3-propanediol (PD) and 1,4-butanediol (BD). LA/DMT/diol feed ratios of 2:1:2, 1:1:2, and 1:2:4 are used in each diol system. It is found that types of the diols play an important role in the properties of the copolyester, where an increase in diol length results in an increase in the copolymers molecular weight, and a decrease in Tg, Tm and crystallinity, when a constant monomer feed ratio is employed. Monomer feed ratio also has a significant effect on properties of the copolymers, where an increase in the aromatic content leads to formation of copolymers with higher molecular weight, longer aromatic block sequence and high aromatic to aliphatic ratio in the chain structure. These, in turn, lead to an increase in Tg, Tm, crys- tallinity and thermal stability of the copolymer samples, and a reduction in their solubility.

Poly(L)lactide (PLLA), aliphatic polyester, and poly(LLA-co-DLLA) copolymers consisting of 2.5, 7.5, 50% of DLLA content were also synthesized. PLLA was successfully electrospun by using 15wt% solution in (1DMF:3CHCl3) mixed solvent. 2.5, 7.5, 50% P(LLA-co-DLLA) copolymers were then spun at 8, 10, and 15wt% concentration in a single chloroform solvent, respective. The lactide-based polymeric nanofibers were characterized by Scanning Electron Microscope (SEM). Smooth surface morphology was observed in nanofibers produced from PLLA and 50% P(LLA-co-DLLA) copolymer. However, surface porosity was observed in the corresponding fibers from 2.5 and 7.5% P(LLA-co-DLLA) copolymers. These nanofibers have high potential for wide range of applications such as filter media, nano-sensor, drug delivery and tissue scaffold, especially, those derived from 2.5 and 7.5% P(LLA-co-DLLA) copolymers which contain high degree of porosity.

Poly(lactic acid-co-ethylene glycol) (PLLA/PEG) copolymers were synthesized and their properties were characterized. The PLLA/PEG/PLLA triblock copolymers were synthesized by ring-opening polymerization from l-lactide (LLA) and PEG macroinitiator. Stannous octoate, Sn(Oct)2 was used as a catalyst. Effects of molecular weight of PEG (600, 2000 and 4000), LLA/OH molar ratios (95:5, 98:2) and a sequence of addition of the reactants on properties of the copolymers were investigated. The triblock copolymers were subsequently used in a production of multiblock copolymers by reacting with a chain-extending agent, hexamethylene diisocyanate (HMDI). Chemical structure and molecular weight of the copolymers were characterized by 1H-NMR, FTIR and GPC. The results showed that molecular weight of triblock copolymers varied from 4,500 to 10,200. After chain extension, multiblock copolymer with molecular weight of 16,490 was produced. Thermal properties of the copolymers were also examined by DSC.

A series of 0-3 composites of lead zirconate titanate (PZT) and nylon77 was prepared with PZT volume fractions between 0.1 and 0.6 using the combined method of solvent casting and hot pressing. Depending upon the volume fraction of PZT, the relative permittivities of the composites were found to be in the range of 200–2000 when measured at the highest practically possible temperature of 200°C. As PZT volume fraction increases, both the relative permittivity and the piezoelectric constant d33 increase. The best combination of electrical properties and flexibility was obtained in 0.5 PZT volume fraction composites. At this composition, the remanent polarization Pr of composites was up to 1000 mC/m2 at the applied electric field of 40 MV/m and it increased with an increase in temperature. The highest piezoelectric strain coefficient d33 was obtained at 17 pC/N in 0.6 PZT volume fraction composites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Lactic acid/ethylene terephthalate copolyesters were synthesized by the standard melt polycondensation of lactic acid (L), ethylene glycol (EG) and dimethyl-terephthalate (DMT). Effects of reaction temperatures and types of catalysts on the structures and properties of the copolymers were examined. In addition, feasibility of promoting the copolymerization process by a novel synthesis step of using thermo-stabilizers was investigated. The results show that a reaction temperature of higher than 180°C is necessary to produce copolymers with appreciable molecular weight. However, degradation was observed when the reaction temperature is higher than 220°C. Triphenyl phosphate (TPP) shows promising results as a potential thermo-stabilizer to minimize this problem. It was found that Sb2O3 and Tin(II) octoate are most effective among 4 types of catalysts employed in this study. 1H-NMR results indicate that copolymers have a random microstructure com- posed mainly of single L units alternately linked with ET blocks at various sequential lengths. The longer ET sequence in the chain structure leads to the increase in melting temperature of the copolymer. TGA results show that the resulting copolymers possessed greater thermal stability than commercially-available aliphatic PLA, as a result of the inclusion of T (terephthalate) units in the chain structure.

PLA-based aliphatic aromatic copolyesters have been synthesized and characterized in order to incorporate the degradability of PLA and good mechanical properties of aromatic species. Synthesis of the copolymers was conducted by polycondensation of lactic acid with dimethyl terephthalate (DMT) and various diols using stannous(II) octoate as a catalyst. Three types of diols with different methylene lengths were employed, i.e., ethylene glycol (EG), propylene glycol (PG) and 1, 4-butanediol (BD). Effects of diols and comonomer molar ratio on the extent of polycondensation reaction and molecular weight of the resulting copolymers were investigated. Diacids and diol ratios of L-lactic acid (LLA), dimethyl terephthalate (DMT) and diol of 1/1/2, 1/2/4 and 2/1/2 were employed. Characterization of chemical structure, molecular weight and thermal and physical properties of the resulting copolymers were conducted by FTIR, NMR, and DSC.

ontrolled-release materials for urea are prepared by spray coating urea granulates with lactic acid based homo- and co-polymers solutions. Percent coating as a function of polymer types, molecular weight, polymer concentration, and dose applications are examined by gravimetric analysis. Percentage of urea dissolution in water of the coated fertilizer is measured by monitoring refractive index of the solutions. Morphology of the polymer coating surfaces is revealed by Scanning Electron Microscopy (SEM). It was found that an amount of cracks and pin-holes, which is dependent on polymer types and molecular weight, plays a significant role in controlling the rate of urea release. Results from urea dissolution test also suggests that the synthesized poly(lactic acid-co-ethylene terephthalate) show urea-holding efficiency comparable to that of commercial PLA, despite its much lower molecular weight, indicating that the copolymer is potentially suitable for this specific application.

Because of their respective advantages, the combination of good material properties of poly(ethylene terephthalate) (PET) and degradability of polylactic acid (PLA) is researched as degradable copolymer for packaging and agricultural applications. Poly(ethylene terephthalate-co-lactic acid) (PET-co-PLA) has been synthesized by employing polycondensation of mixtures of dimethyl terephthalate (DMT), lactic acid (LA) and ethylene glycol (EG), using tin(II) octoate as a catalyst. A chain-extending reagent, hexamethylene diisocyanate (HMDI), was then used in the subsequent step to increase the chain length of the copolymer and improve its mechanical properties for suitable applications. The chemical structure and molecular weight of the copolymers were investigated by FTIR, NMR, and DSC. NMR results indicated the incorporation of lactic acid and PET units in the copolymer chain. Additionally, blends of the resulting copolymer with commercially-available PLA were studied. The blend miscibility was examined by DSC and FTIR spectroscopy.

Naproxen (NPX) drug-loaded magnetic nanoparticles (MNPs) have been prepared in a one-step process utilizing a biocompatible polylactide-grafted-chitosan copolymer. The copolymer serves both as a NPX drug carrier as well as a polymeric surfactant for the synthesis of MNPs without the use of any additional surfactant. Highly stable MNPs with high magnetization in the form of maghemite (γ-Fe2O3) are prepared in aqueous media. Effects of preparation conditions on structures and properties of the copolymer-coated and drug-loaded MNPs are investigated by employing particle size and zeta potential measurements, transmission electron microscopy, vibrating sample magnetometer, X-ray diffraction, Fourier-transform infrared, nuclear magnetic resonance, and confocal Raman spectroscopy. The results show that average particle size (150–300 nm), coating efficiency, and coating structures of the resulting MNPs materials are strongly dependent on MNP/copolymer and MNP/NPX ratios in feed. It is also observed that NPX acts as co-surfactant in the drug-loading process, resulting in different encapsulating structures with the variation in the MNP/copolymer and MNP/NPX ratios. Properties of the MNPs materials can be further optimized for use in specific biomedical applications.

This paper discusses a new ferroelectric polymer with high dielectric constant (>50 at 1K-1M Hz) and large electrostrictive response (~5%) at ambient temperature, which is based on a processable semicrystalline terpolymer comprising vinylidene difluoride (VDF), trifluoroethylene (TrFE), and chlorotrifluoroethylene (CTFE). This VDF/TrFE/CTFE terpolymer was prepared by a combination of a borane/oxygen initiator and bulk polymerization process at ambient temperature. The control of monomer addition afforts the terpolymers with high molecular weight and relatively narrow molecular weight and composition distributions. The incorporated bulky CTFE units homogeneously distributed along the polymer chain seem to reduce the thickness of ferroelectric crystalline domains without destroying the overall crystallinity. This nano-size semicrystalline morphology results in the reduction of ferroelectric-paraelectric (F-P) phase transition to near ambient temperature with a very small energy barrier. Some terpolymers exhibited common ferroelectric relaxor behaviors with a broad dielectric peak that shifted toward higher temperatures as the frequency increased, and a slim polarization hysteresis loop at near the dielectric peak (around ambient temperature) that gradually evolved into a normal ferroelectric polarization hysteresis loop with reduced temperature.