The polyacrylonitrile (PAN)–poly(methyl methacrylate) (PMMA) core–shell nanofiber reinforced dental composites have been investigated for their excellent interface adhesive, and this kind of novel dental composite has the potential for clinical uses such as denture base resin and crown–bridge material.
The first objective of this work was to determine the improving effect of tensile properties by post-drawing PAN–PMMA nanofibers membrane. The second objective was to examine the flexural strength (Fs), flexural modulus (Ey) and work of fracture (WOF) of Bis-GMA/TEGDMA composites reinforced with PAN–PMMA nanofibers.
PAN(core)–PMMA(shell) nanofiber was made by an electrospinning setup with a high-speed rotating rod-like collector. The post-draw process was carried out at 120 °C for 5 min, and all the nanofiber membranes were elongated to the desired elongation ratio (30%, 60% or 100%). Tensile properties and flexural properties of both nanofiber membranes and nanofiber reinforced Bis-GMA/TEGDMA composites were investigated. A scanning electron microscope (SEM) was used to observe the fiber morphology and the fracture surface of the composite. A dynamic mechanical thermal analyzer (DMTA) was employed to determine the dynamic mechanical properties such as tan δ and E ′.
The post-drawing treatment significantly improved the tensile properties and fiber parallelism of nanofiber membranes. The addition of PAN–PMMA nanofibers into Bis-GMA/TEGDMA clearly showed the reinforcement effect; the flexural strength (Fs), flexural modulus (Ey) and work of fracture (WOF) kept rising with the nanofiber mass fraction changing from 0%, 0.6%, 0.8%, 1.0% to 1.2%. The flexural properties of composites reinforced with post-drawn nanofiber were further increased in comparison with those of untreated nanofiber reinforced ones. Also, the SEM observations of the fracture surface of the composites demonstrated good interfacial adhesion between fibers and resin.
The post-drawing treatment was confirmed as a useful method for significantly increasing the tensile strength (673.4%) and tensile modulus (875.3%) of nanofiber membranes. In addition, the composites reinforced with post-drawn PAN–PMMA nanofibers exhibited higher Fs (13.6%), Ey (5.3%) and WOF (30.4%) than those reinforced with as-electrospun PAN–PMMA nanofibers. When 1.2% mass fraction of post-drawn nanofibers were added to Bis-GMA/TEGDMA resin, the Fs, Ey and WOF increased by 51.6%, 64.3% and 152.0%, respectively, compared with neat resin.
Owing to the advantages such as esthetics, fewer health risks and better clinical handling compared with amalgam, 2,2-bis-[4-(methacryloxypropoxy)-phenyl]-propane (Bis-GMA) dental resin has been widely used for decades as a dental restorative material. Considering the relatively low mechanical properties of Bis-GMA resin, lots of research had been done . Nanofibers are believed to have the potential of substantially improving the mechanical properties of Bis-GMA resin for their ultrahigh interfacial area, and some researchers consider that they have the self-tailoring ability to meet demanding mechanical properties , that is, are suitable for various uses such as in denture base resin and crown–bridge material.
The electrospinning technique has been widely used to produce nanofibers from various polymeric solutions or melts . In the electrospinning technique, one electrode is placed into the solution and the other attached to a collector. When the electric force is increased enough to overcome the surface tension of the liquid, a charged jet is ejected and the jet undergoes a process of solvent evaporation, leaving behind a dried polymer fiber which lays itself on a collector. Using the electrospinning method to fabricate nanofiber from polymer blend has been widely reported, but only a few are aimed at dental reinforcement . In the previous work from the authors’ laboratory, Lin Song et al. successfully made the polyacrylonitrile (PAN)–poly(methyl methacrylate) (PMMA) core–shell non-woven nanofiber membrane by co-electrospinning a mixed solution of PAN and PMMA from a single-nozzle. The core–shell structure of PAN–PMMA nanofiber was identified by transmission electron microscope (TEM) with an energy dispersive spectroscopy (EDS) system and the results fully supported this claim. The non-woven PAN–PMMA nanofiber membrane was used to reinforce light-cured Bis-GMA dental resin, the flexural strength (Fs), elastic modulus (Ey) and work of fracture (WOF) increased by 18.7%, 14.1% and 64.8% at most, respectively. They owed the improvement effect to the formation of the semi-IPN structure between the nanofiber and the matrix, which resulted in an ‘in situ nano-interface’ that could provide good interfacial adhesion. Although some achievements had been made in the previous work, there were a few disadvantageous issues of non-woven nanofiber being used as reinforcement. It was reported that the poor orientation of non-woven nanofiber caused a low degree of polymer crystallinity , which would apparently affect the strength of nanofiber membranes. In the meanwhile, high porosity and poor orientation of non-woven nanofiber membranes led to random fiber deposition in the matrix which weakened the reinforcement effect. Besides, the isotropism of non-woven nanofiber affected the formation of perpendicular-loading, and located in tension side, this multidirectional fiber sheet might restrict the strength of composites in any direction .
Recently, to increase the mechanical properties of electrospun nanofiber by post-drawing treatment has attracted the attention of several researchers . It is believed that post-drawing measurement can improve the degree of nanofiber orientation and crystallinity of polymer, which facts are considered as having an important influence on the mechanical properties of nanofiber . Additionally, control of the post-drawing temperature in the vicinity of the glass transition temperature ( T g ) could achieve the maximum effect in reinforcing. But, because of their complicated texture and relatively low strength, it is not practical to stretch the non-woven nanofiber membranes.
In order to meet the greater demand for mechanical performance in dentistry, this research was aimed to further increase the mechanical properties of PAN (core)–PMMA (shell) nanofiber membrane itself and the resultant nanofiber reinforced dental restorative composites. In this study, a rod-like collector was used to take the electrospun nanofiber with high take-up speed, and parallel-aligned nanofiber membranes were obtained. This kind of nanofiber membrane was treated by the post-drawing procedure to get a higher degree of fiber orientation and crystallinity, which was helpful in increasing nanofiber strength . The morphology and tensile properties of PAN (core)–PMMA (shell) nanofiber were examined. Both the post-drawn nanofibers and the as-electrospun nanofibers were used to reinforce Bis-GMA resin, and the flexural properties of the composites were investigated.
Materials and methods
Commercial PAN fibers composed of PAN/methyl acrylate/itaconic acid (93:5.3:1.7 w/w, MW = 100,000 g/mol, Courtaulds Ltd., UK) and PMMA particles (MW = 50,000 g/mol, LG Ltd., Korea) were purchased for electrospinning core–shell structure nanofibers without further purification. Dimethylformamide (DMF), Bis-GMA resin, tri-(ethylene glycol)dimethacrylate (TEGDMA), camphorquinone (CQ), and 2-(dimethylamino)ethylmethacrylate (DMAEMA) were supplied by Aldrich Chemical Co.
Electrospinning of PAN–PMMA core–shell nanofiber and post-drawing treatment
Both PAN fibers (2.6 g) and PMMA particles (0.8 g) were added into 10 ml of DMF, a homogeneous solution obtained under continuous stirring for about 4 h. The shear viscosity of the solution was measured as 3600 mPa s with a digital rotation viscometer (NDJ-5S, Shanghai Precision & Scientific Instrument Ltd., Shanghai, China) under the shear rate of 400 s −1 at 25 °C.
The experimental setup was the same as , and the schematic drawing of the setup is displayed in Fig. 1 (d) . The solution was supplied into a stainless needle by a syringe pump (TOP 5300, Top Corporation, Tokyo, Japan) at 0.4 ml/h −1 flowing rate, subjected to an electric potential 15 kV relative to a vertically rotating grounded rod-like collector wheel. The linear rate of the rod-like collector was 15.7 m s −1 , and the distance from the needle point to the collector was 15 cm. All parallel-aligned nanofiber membranes were vacuum-dried at 60 °C for 48 h to produce membranes of approximately 0.76 mg/cm 2 in weight and 25 μm in thickness.
The nanofiber membranes were stretched along the fiber direction for 5 min in an oven stable at 120 °C. Three elongation ratios (30%, 60% and 100%) were applied.
Morphology of nanofiber
A field-emission SEM (Zeiss Supra 40VP, Carl Zeiss SMT Inc., Wetzlar, Germany) was employed to examine the morphology of PAN–PMMA nanofiber with and without post-drawing treatment. Before examination, all the specimens were sputter coated with a thin layer of gold to allow for better electrical conduction.
A TEM (JEM-3010, JEOL Japan Inc., Tokyo, Japan), operating at 300 kV with a measured point-to-point resolution of 0.17 nm and an energy dispersive X-ray spectroscopy (EDS) system (GENESIS 307, EDAX INC., Mahwah, USA), was used to identify and characterize the core–shell structure of the PAN–PMMA nanofibers.
Fabrication of nanofiber reinforced composites
The nanofiber membranes with 100% elongation were carefully cut into pieces with sizes of 45 mm (along fiber direction) × 5 mm and 25 mm (along fiber direction) × 2 mm. A Teflon mold was used to produce 45 mm × 5.0 mm × 2.0 mm and 25 mm × 2.0 mm × 2.0 mm beam shaped composite specimens for DMTA and mechanical properties testing. The nanofiber parallel-aligned fabric pieces were laminated into the Bis-GMA/TEGDMA (50/50 wt%) resin with photo initiator CQ (mass fraction of 0.5 wt%) and co-initiator DMAEMA (mass fraction of 1 wt%) layer by layer. A vacuum oven was employed to remove the trapped air bubbles between fibers until a transparent state was achieved. Then, all the specimens were light-cured for 1 min using curing light (QHL75, Dentsply International, York, USA) in the yellow-light room to avoid premature curing, and stored at 37 °C for 48 h. The sides of the specimens were carefully polished with 2400 grit silicon carbide paper before tests. The final dimensions of the specimens were measured by Vernier caliper.
The tensile properties of nanofiber membranes were examined by universal material test machine (Instron 1121, Instron European Headquarters, High Wycombe, UK) along the fiber direction. All the samples were cut to the size of 20 mm in length and 5 mm in width. The crosshead speed was 20 mm/min.
The flexural strength (Fs), flexural modulus (Ey) and work of fracture (WOF) of nanofiber reinforced Bis-GMA/TEGDMA composite were investigated using an Instron 1121 universal test machine in a three-point-bending test with a crosshead speed of 1 mm/min and a span of 20 mm, according to ISO 10477. Totally, seven groups of specimens with different mass fractions of nanofiber were tested, which contained 0 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.6 wt% of 100% elongated PAN–PMMA nanofiber and 0.6 wt% of as-spun PAN–PMMA nanofiber. The size of the specimens was 25 mm × 2 mm × 2 mm ( l × b × h ), and the Fs, Ey and WOF were calculated from the following formulae:
Fs = 3 F l 2 b h 2 Ey = l 3 F 1 4 f b h 3 WOF = A b h