苯基-低聚倍半硅氧烷和氨基-低聚倍半硅氧烷对左旋聚乳酸结晶行为及热稳定性的影响

以二氯甲烷为溶剂,将不同相对分子质量的左旋聚乳酸（PLLA）与笼型多面体低聚倍半硅氧烷（POSS）进行溶液共混制备POSS/PLLA复合材料。并通过热台偏光显微镜（POM）、DSC、XRD和TGA对POSS/PLLA复合材料和PLLA的结晶形貌、结晶性能和热稳定性进行了表征。结果表明：POSS/PLLA复合材料的结晶温度升高到110℃左右,结晶能力提高;初始热分解温度和终止热分解温度均增大,最大热分解速率对应的温度在373~379℃之间,热稳定性提高。另外,当POSS含量较低时,成核作用占主导地位;而POSS含量较高时,POSS对PLLA分子链运动的阻碍作用逐渐增强。且POSS的阻碍作用对相对分子质量高的PLLA更明显。观察POSS/PLLA复合材料和PLLA的等温结晶形貌,可以看出明显的十字消光现象和环带球晶形貌,另外,球晶在降温过程中会产生裂纹,这与PLLA的脆性有关。     

Functionalized cellulose nanocrystals as biobased nucleation agents in poly(l-lactide) (PLLA) – Crystallization and mechanical property effects

The important industrial problem of slow crystallization of poly(l-lactide) (PLLA) is addressed by the use of cellulose nanocrystals as biobased nucleation reagents. Cellulose nanocrystals (CNC) were prepared by acid hydrolysis of cotton and additionally functionalized by partial silylation through reactions with n-dodecyldimethylchlorosilane in toluene. Such silylated cellulose nanocrystals (SCNC) were dispersible in tetrahydrofuran and chloroform, and formed stable suspensions. Nanocomposite films of PLLA and CNC or SCNC were prepared by solution casting. The effects of surface silylation of cellulose nanocrystals on morphology, non-isothermal and isothermal crystallization behavior, and mechanical properties of these truly nanostructured composites were investigated. The unmodified CNC formed aggregates in the composites, whereas the SCNC were well-dispersed and individualized in PLLA. As a result, the tensile modulus and tensile strength of the PLLA/SCNC nanocomposite films were more than 20% higher than for pure PLLA with only 1 wt.% SCNC, due to crystallinity effects and fine dispersion.     

Effect of multi-walled carbon nanotubes on the crystallization and hydrolytic degradation of biodegradable poly(l-lactide)

Biodegradable poly(l-lactide) (PLLA)/carboxyl-functionalized multi-walled carbon nanotubes (f-MWNTs) nanocomposites were prepared via solution blending. Scanning electron microscopy observations reveal a fine dispersion of f-MWNTs in the PLLA matrix. The presence of f-MWNTs enhances the crystallization of PLLA in the nanocomposites compared with that of neat PLLA; moreover, the overall crystallization rate of PLLA increases with increasing the f-MWNTs content in the PLLA matrix. The incorporation of f-MWNTs improves the storage modulus of the PLLA/f-MWNTs nanocomposites, with this effect being more pronounced at lower f-MWNTs content. The exciting aspect of this research is the enhanced hydrolytic degradation of PLLA after nanocomposites preparation with f-MWNTs, which may be of great interest for its wide practical application.     

Utilization of polymer degradation to modify electrical properties of poly(l-lactide)/poly(methyl methacrylate)/carbon filler composites

This research attempts to utilize polymer degradability in modifying electrical properties of poly(l-lactide) (PLLA)/poly(methyl methacrylate) (PMMA)/carbon fillers composites. Three kinds of carbon particles, i.e. carbon black, vapor-grown carbon fiber, and carbon nanotube, were compounded with PLLA/PMMA blend, followed by hydrolytic degradation of the composites, resulted in degradation of PLLA molecular chain from the surface of samples, with PMMA and carbon particles remained undegraded. By controlling degradation rate, it was possible to prepare samples with low surface resistivity, yet at the same time exhibited high value of volume resistivity. It was also found that final electrical properties of degraded composites depend on the size and the shape of the fillers.     

Novel nanocomposites based on epoxy resin/epoxy-functionalized polydimethylsiloxane reinforced with POSS

The purpose of the present study is to develop novel nanocomposites based on diglycidylether of bisphenol A (DGEBA) combined with diglycidylether-terminated polydimethylsiloxane (DG-PDMS), reinforced with 10 wt.% (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS). DG-PDMS and POSS compounds were covalently incorporated into DGEBA resin via copolymerization of epoxy groups. The effect of both DG-PDMS and POSS nanoparticles on the curing reaction, glass transition temperature (T_g), thermal stability, hardness and morphology of DGEBA/DG-PDMS ± POSS nanocomposites were studied by DSC, FTIR, DMA, TGA, SEM/EDX, AFM and contact angle measurements. SEM/EDX and AFM results prove that OEP-POSS is well dispersed within DGEBA/DG-PDMS polymer matrix, while MEP-POSS forms large POSS aggregates. The thermo-mechanical properties of POSS based nanocomposites are also in good correlation with morphology features. MEP-POSS based nanocomposite with heterogeneous dispersion of POSS aggregates exhibits lower T_g value and thermal stability in comparison with OEP-POSS nanocomposite which exhibits a nanoscale dispersion of the POSS cages. The obtained T_g of OEP-POSS based nanocomposite increases with 31 °C in comparison with the unreinforced matrix. Moreover, this nanocomposite shows the highest storage modulus (E′) and hardness.     

Development of degradable and bioactive composite as bone implants by incorporation of mesoporous bioglass into poly(l-lactide)

Bioactive composites containing mesoporous bioglass (MBG) and poly(l-lactide) (PLLA) for bone regeneration were fabricated by solution casting method. The results showed that the compressive strength and hydrophilicity of the MBG/PLLA composites significantly improved with the increase of MBG content. In addition, the weight loss ratio of the composites in Tris–HCl solution was obviously enhanced with the increase of MBG content. Moreover, the composite containing MBG could compensate for the decrease of pH value by neutralizing the acidic products from PLLA degradation in the Tris–HCl solution. Furthermore, the MBG/PLLA composites could induce apatite formation on their surfaces after soaked into simulated body fluid (SBF), indicating good bioactivity. In cell culture experiments, the results showed that the composite could enhance cell attachment, proliferation and alkaline phosphatase activity (ALP) of MC3T3-E1 cells, and the improvements were dependent on the MBG content in the composites. In short, the MBG/PLLA biocomposites with improved properties of hydrophilicity, degradability, bioactivity, neutralizing acidic degradable products and good cytocompatibility would be a promising orthopedic implant material for bone repair application.     

Synthesis and characterization of biodegradable poly(l-lactide)/layered double hydroxide nanocomposites

This study reports the preparation and physical properties of biodegradable nanocomposites fabricated using poly(l-lactide) (PLLA) and magnesium/aluminum layered double hydroxide (MgAl-LDH). The MgAl-LDH with molar ratio of Mg/Al = 2 were synthesized by the co-precipitation method. In order to improve the chemical compatibility between PLLA and LDH, the surface of LDH was organically-modified by polylactide with carboxyl end group (PLA–COOH) using ion-exchange process. Then, the PLLA/LDH nanocomposites were prepared by solution intercalation of PLLA into the galleries of PLA–COOH modified LDH (P-LDH) in tetrahydrofuran solution. Both X-ray diffraction data and Transmission electron microscopy images of PLLA/P-LDH nanocomposites indicate that the P-LDHs are randomly dispersed and exfoliated into the PLLA matrix. Mechanical properties of the fabricated 1.2 wt.% PLLA/P-LDH nanocomposites show significant enhancements in the storage modulus when compared to that of neat PLLA. Adding more P-LDH into PLLA matrix induced a decrease in the storage modulus of PLLA/P-LDH nanocomposites, probably due to the excessive content of PLA–COOH moleculars with low mechanical properties. The thermal stability and degradation activation energies of the PLLA and PLLA/P-LDH nanocomposites can also be discussed.     

Interfacial properties and impact toughness of methylphenylsilicone resin composites by chemically grafting POSS and tetraethylenepentamine onto carbon fibers

Octaglycidyl polyhedral oligomeric silsesquioxane (gly-POSS) was successfully grafted on carbon fibers (CFs) surface to enhance interfacial properties and impact toughness of CFs reinforced methylphenylsilicone resin (MPSR) composites. After gly-POSS modification, POSS grafted CF (CF-POSS) with many epoxy functional groups was modified with tetraethylenepentamine (TEPA) to further enhance the interfacial strength. Atomic force microscopy (AFM) images showed that POSS and TEPA were grafted onto CFs surface uniformly and the surface roughness enhanced obviously. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the chemical bonding nature between CFs and POSS, as well as between POSS and TEPA. POSS and TEPA modification could increase the fiber polarity, wettability and surface energy significantly. The interlaminar shear strength (ILSS) and impact toughness of composites showed a dramatic improvement, especially for grafting with POSS and further with TEPA (CF-POSS-TEPA). Additionally, the reinforcing and toughening mechanisms were also analyzed. Meanwhile, single fiber tensile strength (TS) had no decrease after modification.     

Preparation and characterization of polycaprolactone/forsterite nanocomposite porous scaffolds designed for bone tissue regeneration

Biocomposite scaffolds made from polymers and bioceramics can provide the mechanical structure necessary for osteoinductivity in the growth of new bone. The aim of this research was to investigate the properties of a novel nanocomposite scaffold made from a combination of polycaprolactone (PCL) and forsterite nanopowder which could find use in bone tissue engineering applications. The scaffold itself was fabricated by a method of solvent casting and particle leaching. The effect of forsterite content on the mechanical properties, bioactivity, biodegradability, and cytotoxicity of the scaffolds was investigated. Significant improvement in the mechanical properties was observed in the nanocomposite scaffolds as compared to that seen in the pure PCL scaffolds. Bioactivity was also observed in the nanocomposite scaffolds, a trait which was not present in the pure PCL scaffolds. Biodegradation assay indicated that the addition of forsterite nanopowder could modulate the degradation rate of PCL. In vitro tests of cytotoxicity and osteoblast proliferation showed that the nanocomposite scaffolds were non-cytotoxic, thereby allowing cells to adhere, grow, and proliferate on the surface of these scaffolds. The results obtained in this experiment suggest that the combination of PCL with forsterite nanopowder can be used to form scaffolds suitable for use in bone tissue engineering. The exact material behavior required can be adjusted through variation of the ratio between PCL and forsterite nanopowder used to form the scaffold.     

TriSilanolPhenyl POSS–polyimide nanocomposites: Structure–properties relationship

Polyhedral Oligomeric Silsesquioxane (POSS)–polyimide (PI) thin films were synthesized from pre-mixed solution of oxydianiline–pyromellitic dianhydryde (ODA–PMDA) and TriSilanolPhenyl (TSP) POSS. POSS–PI polymerization reaction kinetics was studied using Fourier Transform Infrared (FTIR) spectroscopy. The POSS–PI films were then investigated by tensile tests, followed by surface morphology examination using Atomic Force Microscopy (AFM) and Environmental Scanning Electron Microscopy (ESEM). An interdisciplinary approach was applied for establishing a relation between POSS–PI composites chemical microstructure properties and failure mechanisms. Inter molecular POSS–POSS interaction by either phase separation, or chemical POSS–POSS condensation reaction were observed as key factors, affecting the nanocomposite mechanical properties via formation of aggregates. The amount and density of these aggregates were shown to be composition dependent. A model based on formation and coalescence of voids during tensile tests was suggested for understanding the effect of the POSS content on the POSS–PI mechanical response.     

Electrospun aligned PLLA/PCL/functionalised multiwalled carbon nanotube composite fibrous membranes and their bio/mechanical properties

Aligned poly(L-lactide) (PLLA)/poly(ε-caprolactone) (PCL)/functionalized multiwalled carbon nanotube (F-MWNT) composite fibrous membranes were fabricated by electrospinning. Their morphology, mechanical properties, in vitro degradation and biocompatibility were studied. With a collector rotation speed of 3000 rpm, the electrospun fibers are highly aligned and the F-MWNTs are oriented along the fiber axis, reinforcing the electrospun fibrous membranes. When the F-MWNTs are incorporated, the PLLA/PCL/F-MWNT composite fibers become thinner due to the increased electrical conductivity. However, when the F-MWNTs are increased to 3.75 wt.%, the higher viscosity and aggregation of F-MWNTs have lead to the formation of beads and a wider diameter distribution in the electrospun fibers. Also, the electrospun fibers having smaller diameter, larger porosity and lower crystallinity induced by F-MWNTs have improved the bio-degradation of the PLLA/PCL/F-MWNT fibrous membranes, which have no toxic effects on the proliferation of adipose-derived stem cells.     

Effect of the POSS–Polyimide nanostructure on its mechanical and electrical properties

Nanocomposite films consisted of Polyhedral Oligomeric Silsesquioxane (POSS) filler in a Polyimide (PI) matrix were prepared. The effect of the nanocomposites’ structure on its mechanical and electrical properties was evaluated with respect to survival in the low Earth orbit (LEO) environment. The POSS–PI structure consists of POSS nano-aggregates formed in the bulk and on the surface. The aggregates’ size and distribution are POSS content-dependant. The fracture mechanism during hypervelocity impact at extreme temperature conditions was studied. The hypervelocity impacts of the POSS–PI films result in a brittle fracture, compared to ductile fracture in the case of PI, and in formation of radial cracks. A model based on formation and coalescence of voids around the aggregates, when load is applied, is suggested to explain the effect of the POSS content on the POSS–PI fracture mechanism. The size and density of the POSS aggregates also affect the nanocomposite’s volume electrical resistivity. An inverse dependence exists between the POSS aggregates’ surface density and the nanocomposites’ volume electrical resistivity.     

Effect of the content of hydroxyapatite nanoparticles on the properties and bioactivity of poly(l-lactide) – Hybrid membranes

Poly(l-lactide)/hydroxyapatite, PLLA/HA, composite membranes for bone regeneration with different concentrations of nanoparticles have been prepared and their physicochemical properties and bioactivity have been determined. Hydroxyapatite nanoparticles act as nucleating agent of the poly(l-lactide) crystals, as detected by DSC, and as reinforcing filler, as proven by the monotonous increase of the elastic modulus of the microporous membranes with increasing nano-filler content. The bioactivity, which regards to the use of these materials in bone regeneration, was tested by immersing the samples in a simulated body fluid, SBF. A faster deposition of a biomimetic apatite layer was observed as increases the content of hydroxyapatite nanoparticles, thus membranes with a 15% (w/w) of hydroxyapatite particles (relative to PLLA weight) present a homogeneous layer of hydroxyapatite on the surface of their pores after 7 days of immersion in SBF. An especial emphasis has been made on the influence of a plasma treatment on the bioactivity of the membranes. With this aim, the membranes were submitted to a plasma treatment previously to their immersion in a simulated body fluid. It has been observed that the surface of a PLLA membrane after 21 days of immersion in SBF is still not completely covered by hydroxyapatite whereas the same sample treated with plasma show a smooth layer of biomimetic hydroxyapatite. The increase of bioactivity achieved with this treatment was less important in high hydroxyapatite content composites.     

Surface functionalisation of bacterial cellulose as the route to produce green polylactide nanocomposites with improved properties

The effect of surface functionalisation of bacterial cellulose nanofibrils (BC) and their use as reinforcement for polylactide (PLLA) nanocomposites was investigated. BC was functionalised with various organic acids via an esterification reaction. This rendered the otherwise hydrophilic BC hydrophobic and resulted in better compatibility (interfacial adhesion) between PLLA and BC. A direct wetting method, allowing the determination of the contact angle of polymer droplets on a single BC nanofibre, was developed to quantify the interfacial adhesion between PLLA and functionalised BC. It was found that the contact angle between PLLA droplets and functionalised BC decreased with increasing chain lengths of the organic acids used to hydrophobise BC. A novel method to compound BC with PLLA based on thermally induced phase separation (TIPS) to yield a dry form of pre-extrusion composite was also developed. The mechanical properties of the surface functionalised BC reinforced PLLA nanocomposites showed significant improvements when compared to neat PLLA and BC reinforced PLLA. The thermal degradation and viscoelastic behaviour of the nanocomposites were also improved over neat PLLA.     

Filling behavior,morphology evolution and crystallization behavior of microinjection molded poly(lactic acid)/hydroxyapatite nanocomposites

In this paper, the filling behavior, morphology evolution, crystallization behavior, thermal stability and mechanical property of poly(lactic acid) (PLA)/hydroxyapatite (HA) nanocomposite under microinjection molding conditions were systematically investigated. The comparison between micropart and macropart of PLA/HA nanocomposite was also conducted. Results showed that in the four stages occurring in the microinjection molding process, the mold cavity filling stage is an extremely rapid process and injection speed influences the filling behavior much more significantly than mold temperature. The remarkably enhanced shear force field generated under microinjection molding conditions proves to be beneficial to formation of highly oriented PLA matrix self-fibrillating structure, improvement of HA filler dispersion and enhancement of interfacial combination. Formation of such a highly oriented structure could lead to the remarkable difference in both crystallization behavior and mechanical property between micropart and macropart. The PLA/HA nanocomposite micropart possessed a significantly enhanced mechanical property and showed a good application prospect.     

Environmental degradation of epoxy–organoclay nanocomposites due to UV exposure. Part I: Photo-degradation

The environmental degradation mechanisms of epoxy–organoclay nanocomposites due to accelerated UV and moisture exposure are studied. Various characterisation tools, including FTIR, SEM, XRD and XRF analyses, were used to evaluate the effects of clay content on the progressive changes in chemical element, topography and colour of the nanocomposite. It is found that microcracks started to appear on both the neat epoxy and nanocomposite surface after about 300 h of UV exposure. The nanocomposite exhibited thicker and shallower cracks with a less degree of discoloration than the neat epoxy due to the diffusion barrier characteristics of organoclay with high aspect ratio. The presence of transition metal ions along with low-molecular-weight organic modifiers in organoclay, however, accelerated the degradation of polymer, counterbalancing the above ameliorating barrier properties of clay. FTIR analysis indicated that photo-degradation generated carbonyl groups by chain scission and the rate was slightly higher for the nanocomposites than for the neat epoxy. While moisture further accelerated the photo-degradation process through the enhanced mobility of free radicals and ions, the organoclay could limit the deteriorating effect of moisture, resulting in much better overall resistance to photo-degradation in the presence of moisture for the nanocomposite than the neat epoxy.     

Self-reinforcement and hydrolytic degradation of amorphous lactic acid based poly(ester-amide), and of its composite with sol-gel derived fibers

The self-reinforcing and hydrolytic degradation of an amorphous poly(ester-amide) (PEA) based on lactic acid have been studied and compared with those of poly-L-lactide (PLLA). The studied PEA-rods were self-reinforced (SR) by solid-state die drawing resulting double shear strength. The hydrolytic degradation of PEA was studied during exposure to phosphate buffered saline at pH 7.4 and at 37 °C for 18 weeks. The degradation and mechanical properties of PEA were also followed in a self-reinforced composite structure consisting of PEA and sol-gel derived SiO₂-fibers (SGF, 8 wt %). The hydrolytic degradation of the SR-PEA-rods with and without SG-fibers was significantly faster than that of SR-PLLA-rods. The weight average molecular weight (M _w) of PEA decreased by 90% from the initial M _w during the first 6 weeks in hydrolysis, when the M _w of the PLLA decreased by 10%.     

Solidification behavior of PLLA/nHA nanocomposites

As part of a broader effort to establish processing-structure–property relationships in PLLA/nHA, which is currently under consideration for bioresorbable scaffolds for bone repair, hot stage optical microscopy and differential scanning calorimetry have been used to investigate the solidification behavior of a series of medical grade PLLA/nHA nanocomposites. The presence of the nHA resulted in an increase in the number of spherulites per unit volume during isothermal crystallization, but there was a substantial decrease in the spherulite growth rate with increasing nHA content in the temperature range 100–130 °C, argued to be associated with a significant increase in the melt viscosity in the presence of the nHA. The consequences for the global solidification rates and the phase structure of the PLLA/nHA nanocomposites are discussed.     

纳米纤维素晶须增强增韧聚(L-乳酸)复合材料的制备与表征

通过酸解法制备了具有纳米尺寸和一定长径比针棒状的纳米纤维素晶须(NCW),利用NCW表面的羟基引发L-丙交酯开环聚合,合成了表面接枝聚(L-乳酸)(PLLA)链段的接枝纤维素晶须(g-NCW);采用溶液浇铸法制备了PLLA膜以及不同配比的NCW/PLLA和g-NCW/PLLA复合膜。对接枝改性前后的NCW的形貌与性能进行了表征,研究了复合膜的形貌、结晶性能、热稳定性、亲/疏水性和拉伸性能。结果表明:NCW的形貌与结晶性能在接枝改性后变化不大,但在乙醇和PLLA溶液中的分散性得到明显改善;当NCW与L-丙交酯的物质的量之比为1∶5时,g-NCW表面PLLA链段的接枝率约为23.61%。NCW和g-NCW作为异相成核剂,显著提高了PLLA基体的结晶速率;并且,加入晶须改善了材料的亲水性和热稳定性。添加一定量的NCW和g-NCW到PLLA中,可有效增强增韧PLLA基体;随着晶须含量增加,复合膜的拉伸强度和断裂能先增大后下降;当NCW和g-NCW的质量分数为5%时,NCW/PLLA和g-NCW/PLLA复合膜的拉伸强度和断裂能分别达到22.02 MPa和20.01 MPa以及102.39J/m~3和117.83J/m~3,均达到最大值。由于g-NCW在基体中良好的分散性以及与基体间的界面结合,g-NCW/PLLA复合膜的拉伸强度和韧性明显优于相应的纯PLLA和NCW/PLLA膜。     

Water absorption and solubility of PHBHV/HA nanocomposites

A thorough understanding of the influence of bioceramic phase on the water absorption and solubility of biomaterial is of importance in tailoring its degradation and the formation of bone-like apatite for clinical implant applications in a wet environment. The objective of this study was to characterize and quantify the water transport properties and solubility of biomaterial incorporating poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) and nano-hydroxyapatite (nano-HA) modified with a silane coupling agent. Solubility and transport parameters such as diffusion, permeability, and sorption coefficients were determined at three different temperatures using the weighing method. When the environmental temperature reached 60 °C, the water uptake of the nanocomposite reaches equilibrium after a normally fast absorption process, and then decreases as the immersion time is prolonged due to the solubility of the material. Moreover, this phenomenon becomes more significant with increasing the volume fraction of nano-HA. Compared to those for the base resin, the diffusion coefficients for the nanocomposite decrease, whereas the sorption coefficients and the solubility show an opposite tendency. All of the transportation parameters are temperature sensitive and obey the Arrhenius or the van’t Hoff relationship. Results from thermodynamic analysis imply that when using a high filler loading level (20 vol%) the sorption of the nanocomposite is mainly dominated by a Langmuir sorption mode giving an exothermic process.