J. C. Viana

The thermomechanical environment and the microstructure of an injection moulded polypropylene copolymer

The microstructure of an injection moulding propylene copolymer is varied through systematic changes on the processing conditions (melt and mould temperatures and injection flow rate). The skin-core structure was characterised by several experimental techniques. The skin ratio was assessed by polarised light microscopy. The morphological features of the skin layer (level of crystalline phase orientation, degree of crystallinity, β-phase content and double texture) were evaluated by wide-angle X-ray diffraction. The core features (degree of crystallinity and lamella thickness) were assessed by differential scanning calorimetry. The thermomechanical environment imposed during processing was characterised by mould filling simulations. The thermal and shear stress levels were evaluated by a cooling index and the wall shear stress. The results show the relationship between these and the microstructural features. The microstructure development is then interpreted considering the constrictions imposed during processing, being assessed by thermomechanical indices. Furthermore, the direct connections between these indices and the degree of crystallinity of the core and the level of orientation of the skin are verified.

Dynamic mechanical behavior of starch-based scaffolds in dry and physiologically simulated conditions: Effect of porosity and pore size

The three-dimensional scaffolds of a blend of starch and poly(L-lactic) acid, SPLA70, were produced using compression molding of polymer/salt mixture followed by leaching of salt. One series of scaffolds were prepared with varying polymer-to-salt ratio while keeping the salt size constant, and the other series of scaffolds were prepared with varying salt sizes while keeping the polymer-to-salt ratio constant. The X-ray microcomputed tomography and scanning electron microscopy assay were used to analyze the porous morphologies, porosity and distribution of porosity of the porous scaffolds. Salt-free and integrated SPLA70 scaffolds with porosities ranging from 74% to 82% and pore sizes of 125-250 to 500-1000 microm can be fabricated using the present fabrication technique. The water uptake of the SPLA70 scaffolds increases with increasing porosities and also with increasing pore size. In dry state, the storage modulus decreases with increasing porosity and also with increasing pore size. The normalized modulus values are related to normalized density of the scaffolds by a power-law function with an exponent between 2 and 3. For the immersed scaffolds under physiological conditions, the storage modulus was less dependent on porosity and pore size. However, the loss factor increased significantly compared with dry state measurements. The present study clearly shows that the mechanical performance of porous polymeric constructs in dry and in immersed state is completely different, and for comparison with biomechanical performance of tissues, the tests should ideally be performed in immersed state.

Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene–butadiene–styrene/multiwall carbon nanotubes composites

Composites of styrene–butadiene–styrene (SBS) block copolymer with multiwall carbon nanotubes were processed by solution casting to investigate the influence of filler content, the different ratios of styrene/butadiene in the copolymer and the architecture of the SBS matrix on the electrical, mechanical and electro-mechanical properties of the composites. It was found that filler content and elastomer matrix architecture influence the percolation threshold and consequently the overall composite electrical conductivity. The mechanical properties are mainly affected by the styrene and filler content. Hopping between nearest fillers is proposed as the main mechanism for the composite conduction. The variation of the electrical resistivity is linear with the deformation. This fact, together with the gauge factor values in the range of 2–18, results in appropriate composites to be used as (large) deformation sensors.

Development of porous lamellar poly(L-lactic acid) scaffolds by conventional injection molding process.

A novel fabrication technique is proposed for the preparation of unidirectionally oriented, porous scaffolds by selective polymer leaching from lamellar structures created by conventional injection molding. The proof of the concept is implemented using a 50/50 wt.% poly(L-lactic acid)/poly(ethylene oxide) (PLLA/PEO) blend. With this composition, the PLLA and PEO blend is biphasic, containing a homogeneous PLLA/PEO phase and a PEO-rich phase. The two phases were structured using injection molding into well-defined alternating layers of homogeneous PLLA/PEO phase and PEO-rich phase. Leaching of water-soluble PEO from the PEO-rich phase produces macropores, and leaching of phase-separated PEO from the initially homogeneous PLLA/PEO phase produces micropores in the lamellae. Thus, scaffolds with a macroporous lamellar architecture with microporous walls can be produced. The lamellae are continuous along the flow direction, and a continuous lamellar thickness of less than 1 microm could be achieved. Porosities of 57-74% and pore sizes of around 50-100 microm can be obtained using this process. The tensile elastic moduli of the porous constructs were between 580 and 800 MPa. We propose that this organic-solvent-free method of preparing lamellar scaffolds with good mechanical properties, and the reproducibility associated with the injection molding technique, holds promise for a wide range of guided tissue engineering applications.

Full elastic constitutive relation of non-isotropic aligned-CNT/PDMS flexible nanocomposites

The elastic response of vertically aligned-carbon nanotube/polydimethylsiloxane (A-CNT/PDMS) nanocomposites is presented in this study and related to the underlying aligned-CNT morphology. Multiwalled carbon nanotubes (MWCNTs) at 1% Vf are embedded in a flexible substrate of PDMS to create a flexible polymer nanocomposite (PNC). The PNC properties are evaluated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and tensile mechanical tests, and the full linearly elastic constitutive relation is established for such a PNC. The results suggest that the CNTs retain the alignment after wetting and curing of PDMS. PDMS is significantly modified by the reinforcing aligned-CNT fibers, demonstrating non-isotropic (as opposed to the isotropic neat PDMS) elastic properties all different from PDMS (Youngs modulus of 0.8 MPa), including an anisotropy ratio of 4.8 and increases in the modulus of A-CNT/PDMS over PDMS by more than 900% and 100%, in the CNT longitudinal and transverse directions, respectively. This study reports the first full constitutive relation that may be useful in modeling PNCs as composites or as elements of hierarchical nanoengineered composites, particularly PDMS-CNT PNCs are envisioned as elements in biomedical devices such as pressure transducers and energy harvesters.

The effect of the skin thickness and spherulite size on the mechanical properties of injection mouldings

In this work are studied the relationships between the microstructure and the mechanical properties of an injection moulded propylene-ethylene copolymer. Distinct microstructures were obtained by processing, through a moulding programme that includes the variation of the injection and the mould temperatures and the injection flow rate. They were characterized by the skin ratio (measured by polarised light microscopy) and the spherulite size (evaluated by small angle light scattering system). Tensile tests were carried out at two different constant loading velocities: 2 mm/min (3.33 × 10−5 m/s) and 3 m/s, in order to assess the initial modulus, the yield stress, the strain and the energy at break. The results are presented in terms of the relationships between the chosen microstructural parameters and the selected tensile properties. The skin thickness is evidenced as an important microstructural feature. The role of the core spherulite size is secondary or even negligible. The results also show that other microstructural parameters must be considered to establish more general microstructure-properties relationships.

Effects of the strain rate and temperature in stress–strain tests: study of the glass transition of a polyamide-6

Abstract In this work new insights are presented on the measurement of the tangent and secant moduli from stress–strain curves in polymeric systems. Expressions for the strain-rate and strain dependence of both moduli are derived for systems characterised by a distribution of relaxation times. The equivalent frequency of the stress–strain experiments is shown to be dependent on the strain rate and on the strain at which the measurements are carried out. Such considerations enable using quasi-static tensile stress–strain tests to study relaxational processes in polymeric materials. The tensile behaviour of a 30% glass fibre reinforced polyamide 6 was characterised at different strain rates and temperatures, covering the glass transition region. A master curve of the tangent modulus as a function of strain rate was successfully constructed by simple horizontal shifting of the isothermal data. The temperature dependence of the shift factors was well described by the WLF equation. It was also possible to fit the master curve considering a polymeric system with a distribution of relaxation times, relevant parameters such as the KWW β parameter being extracted. The results were found to be consistent with dynamic mechanical analysis results.

Characterization of PET nanocomposites with different nanofillers

This study investigates the influence of various nanofillers of different shapes and sizes on the properties of PET nanocomposites. PET was reinforced with 0.3 wt.% of different nanoreinforcements, namely: (i) 1D platelet-like shape of organo-modified layered silicates (montmorillonite) (platelet size approx. 1 x 200 nm) with average agglomerate size of: (a) 30 μm and (b) 8 μm; (ii) 3D spherical shape particles of titanium oxide with an average size of 21 nm and (iii) 3D spherical shape silica with an average particle size of 12 nm. PET nanocomposites were prepared by melt blending in an asymmetric batch minimixer followed by compression moulding process. The effect of nanofillers upon thermal, mechanical and structural properties in comparison to the neat PET are discussed.

Mold for Manipulation Microstructure Development

Abstract The control of microstructure development in injection molding had attracted several important studies in the recent years, being well established that the thermo-mechanical environment imposed to the polymer melt depends on the material properties, molding geometry and processing conditions. An entirely new design of an injection mold RCEM (Rotation, Compression and Expansion Mold) was developed and implemented, allowing for a wide variation of the filling and packing conditions in a flat disk geometry. One of the cavity walls (the ejection side) can rotate or move with a linear movement during the injection and holding stages. This enables several filling sequences and the variation of the cavity thickness. Two electric servomotors are use to directly drive those movements. Two temperatures and pressure sensors allow for the on-line process monitoring. Several examples of the potential uses of the RCEM are shown for the cases of amorphous and semicrystalline polymers. Results obtained with short-fiber reinforced polypropylene are also presented.

Melt blending and characterization of carbon nanoparticles-filled thermoplastic polyurethane elastomers

In this work, thermoplastic polyurethane (TPU) elastomers reinforced with carbon nanosized particles were produced by a special melt blending technique. A TPU was melt blended with high-structured carbon black and carbon nanofibres (1 wt%). A miniature asymmetric batch mixer, which applies high shear levels to the melt, ensured good particles dispersion. The TPU material systems were then thoroughly characterized using thermogravimetric analysis, differential scanning calorimetry, tensile mechanical testing, electrical resistance measurements and flammability tests. The different nanofillers exhibited different influences on the TPU properties, these materials featuring interesting and improved multifunctional behaviours, with high propensity for large deformation sensors applications.