Polypropylene (PP) with high melt flow index (MFI) or low molecular weight, although desired in melt spinning for enhanced productivity, is difficult to be spun into high-strength fiber using the standard process where extensive jet stretching is applied. In this work, a processing route involving minimal jet stretch has been explored. A two-stage hot drawing procedure in the solid state was found to be suitable for producing high-strength fiber from low-molecular-weight PP with an ultrahigh MFI of 115 g/10 min. Fibers produced achieve a maximum tensile strength and Young's modulus of approximately 600 MPa and 12 GPa, respectively. The melt temperature of the fiber reached 170.8°C, approximately 5°C higher than that of the original resin. Wide-angle X-ray diffraction (WAXD) study shows that the stable α-monoclinic crystalline structure is developed during the drawing process. A well-oriented crystalline structure along the fiber axis is generated, having a crystalline orientation factor as high as 0.84. POLYM. ENG. SCI., 56:233–239, 2016. © 2015 Society of Plastics Engineers

Gel spinning of UHMWPE fibers using low molecular weight polybutene (PB) as a new spin solvent was investigated. A 98/2 wt% PB/UHMWPE gel exhibits a melting temperature around 115°C and shows large-scale phase separation upon cooling the solution to room temperature. The resulting precursor fiber from this gel was hot-drawn to a ratio of 120, yielding a fiber with tensile strength of 4 GPa and Young's modulus of over 150 GPa. Wide-angle X-ray diffraction indicates good molecular orientation along the fiber axis. The results also demonstrate the potential to further improve the mechanical properties. With respect to the gel spinning industry, this new solvent has a number of advantages over paraffin oil and decahydronaphthalene, and holds a promise of greatly improving the process efficiency. POLYM. ENG. SCI., 56:697–706, 2016. © 2016 Society of Plastics Engineers

A new method for fast solvent removal in gel spinning was investigated. Instead of solvent evaporation or coagulation as conventionally used, the new method involves mechanically twisting the gel-fiber along the fiber axis. By removing the majority of solvent in the gel-fiber by mechanical twisting not only the emission of solvent vapor and the production of waste solvent mixture or coagulation byproducts are minimized but also the fiber production rate is significantly increased. The new solvent removal method was demonstrated through gel spinning of high-strength ultrahigh-molecular-weight polyethylene fibers using both volatile and nonvolatile spin solvents. Approximately 90% of the spin solvent was removed by a single-step twisting process and the resulting fiber retained the high mechanical properties conventionally obtained. A mechanistic model was developed for estimating the solvent removal as a function of twisting. With respect to the gel spinning industry, the new solvent removal method holds a promise of simplifying the solvent removal and recovery steps and improving the production rate, leading to more efficient and effective gel spinning processes. POLYM. ENG. SCI., 55:745–752, 2015. © 2014 Society of Plastics Engineers

A new twist-gel spinning process for ultrahigh molecular weight polyethylene fibers is demonstrated which significantly increases the extraction rate of nonvolatile spin solvent while simultaneously reducing the consumption of extraction solvent by more than 75%. Applying twist to the gel fiber enables it to be directly hot-drawn, allowing conventional solvent extraction to proceed significantly faster. While solvent extraction effectiveness is largely enhanced, the new process does not show reduced fiber properties. The tensile strength, Young's modulus, surface morphology, and geometry are relatively unaffected when compared to fibers produced using the conventional gel-spinning process. The new twist-gel spinning process is expected to improve the processing efficiency of gel-spun high-strength fibers, promoting broad expansion of these high performance fibers into applications that were previously prohibitive due to extremely slow production. POLYM. ENG. SCI., 55:1389–1395, 2015. © 2015 Society of Plastics Engineers

Composite polystyrene foam with a honeycomb-like barrier structure was processed from an expandable aqueous suspension. Optical observations confirmed the formation of such a unique structure that encapsulates each expanded polystyrene microsphere in the resulting foam. The suspension viscosity was found to highly influence the foam morphology. Results from mechanical tests showed that the existence of the barrier structure can considerably improve the mechanical performance of the composite foam. Fire retardation tests demonstrated that the barrier structure can effectively stop the fire path into the foam, suppress toxic smoke generation, and maintain structural integrity, leading to improved fire resistance in comparison with the neat polystyrene foam. POLYM. ENG. SCI., 55:1494–1503, 2015. © 2015 Society of Plastics Engineers

In this work, a microwave expansion process to produce thermoset-matrix syntactic foam containing thermoplastic foam beads was designed and developed. Expandable polystyrene (EPS) microspheres and epoxy resin were chosen as a model material system. This process is featured with a capability to effectively expand EPS microspheres in syntactic foam with high EPS loading. The resin viscosity and specific microwave energy are found to be the two primary control parameters determining the process window. Mechanical characterization showed that the specific flexural strength and modulus of the syntactic foam are similar to those of the neat epoxy. By comparison, the flexural moduli over density squared or cubed of the foam are much higher, especially at high EPS loadings, than those of the neat resin. The foamed EPS microspheres can also effectively toughen the syntactic foam, preventing propagation of cracks. Furthermore, the microwave expansion process was found to be capable of molding syntactic foam parts of relatively sophisticated geometry with smooth surfaces. POLYM. ENG. SCI., 55:1818–1828, 2015. © 2014 Society of Plastics Engineers

In conventional hot embossing, a thermoplastic polymer undergoes phase transitions in liquid, semi-solid, and solid states through cyclic heating and cooling. This paper, in contrast, describes the development of a constant-temperature embossing process and compares its characteristics against standard hot embossing. The new process utilizes the crystallizing nature of supercooled polymer films to obtain the necessary phase transitions. By softening and crystallizing the supercooled polymer at the same temperature, the embossing and solidification stages can be carried out isothermally without a cooling step. PET, due to its relatively slow crystallizing kinetics, was chosen as a model material for this study. The embossed films with microgroove patterns of different sizes and aspect ratios were characterized for their replication fidelity and accuracy. For supercooled PET films, constant-temperature embossing with high replication quality and acceptable demolding characteristics was achieved in a large processing temperature window between T_g and T_m of PET. A parametric process study involving changes of the embossing temperature and embossing time was conducted, and the results indicated that the optimal process parameters for constant-temperature embossing can be derived from the crystallization kinetics of the polymer. The removal of thermal cycling is a major advantage of constant-temperature embossing over conventional hot embossing and represents an important process characteristic desired in industrial production. POLYM. ENG. SCI., 54:1100–1112, 2014. © 2013 Society of Plastics Engineers

A microsphere templating process was recently developed for fabrication of open-cell porous elastomer. The material contained a unique cocontinuous structure having micropores interconnected by microchannels. In this work, a follow-up study was conducted to investigate the mechanical behavior of such porous materials. Polysiloxane was chosen as a model material. The deformation characteristics and mechanical properties of the porous elastomer under tension and compression were then studied. For both tensile and compressive tests, the mechanical properties measured were found to largely deviate from those calculated by the additive rule assuming affine deformation. This nonaffine mechanics was further verified by microscopic observations. Cyclic loading and unloading tests were also performed to study the hysteresis of the material. In comparison with the solid elastomer, the hysteresis of the porous elastomer was considerably higher and more sensitive to the strain rate. An attempt was further made to fit the stress-strain curve using existing hyperelastic models, and the results showed that the Arruda–Boyce model, in general, fit both the solid and porous polysiloxane very well. POLYM. ENG. SCI., 54:1512–1522, 2014. © 2013 Society of Plastics Engineers

This article describes a new gel-spinning process for making high-strength poly(ethylene oxide) (PEO) fibers. The PEO gel-spinning process was enabled through an oligomer/polymer blend in place of conventional organic solvents, and the gelation and solvent-like properties were investigated. A 92/8 wt% poly(ethylene glycol)/PEO gel exhibited a melting temperature around 45°C and was highly stretchable at room temperature. Some salient features of a gel-spun PEO fiber with a draw ratio of 60 are tensile strength at break = 0.66 ± 0.04 GPa, Young's modulus = 4.3 ± 0.1 GPa, and a toughness corresponding to 117 MJ/m³. These numbers are significantly higher than those previously reported. Wide-angle x-ray diffraction of the high-strength fibers showed good molecular orientation along the fiber direction. The results also demonstrate the potential of further improvement of mechanical properties. POLYM. ENG. SCI., 54:2839–2847, 2014. © 2014 Society of Plastics Engineers

In contrast to the immense literature in porous material generation, micropatterning of porous devices remains a technical challenge. In this study, a new process for micropatterning of porous structures with a controllable morphology was developed and investigated. This process combines polymer melt blending, hot embossing, and in-mold annealing for geometrical pattern transfer and simultaneous morphological control. A special effort was made to generate a microgroove pattern with an open pore structure. Parametric experimental studies were conducted on stamps with different feature sizes under different processing conditions. The results demonstrated the feasibility and the versatility of the proposed technique in fabricating micropatterned porous structures. By varying the geometrical and boundary conditions during in-mold annealing, micropatterns with graded porous structures were demonstrated. © 2012 Wiley Periodicals, Inc. Adv Polym Techn 32: E166–E179, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21260

In this work, a solvent-free microsphere-templating process was developed to fabricate porous elastomers. The process starts with preparation of a porous wax template by thermal sintering at a temperature close to the wax melting temperature. Then, low-viscosity monomers for elastomeric polyurethane were cast into the wax template. Finally, after casting and curing of the elastomer, the wax component was removed by mechanical forces (centrifuge or squeezing) to produce a porous elastomer. With this process, polyurethane scaffolds with a co-continuous porous structure were successfully prepared. The optimal sintering temperature for the wax template was attained about 4°C below the wax peak melting temperature, leading to a stable process for joining wax microspheres into a three-dimensional porous template. Mechanistic approaches were found to be effective for extracting wax in the cast polyurethane; more than 90% of wax can be simply removed by mechanical squeezing above the wax melting temperature. After wax removal, the polyurethane demonstrated a morphology with micropores interconnected by microchannels, indicating the interconnectivity of the porous structure. Tensile tests showed that the resulting porous material is highly deformable and elastic, exhibiting typical properties of polyurethane elastomers. © 2012 Wiley Periodicals, Inc. Adv Polym Techn 2013, 32, 21330; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21330

Because of its slowly crystallizing nature, poly(ethylene terephthalate) (PET) can be supercooled into an amorphous glass by rapid quenching. Upon reheating between T_g and T_m, the amorphous PET are subjected to two competing processes: rubber softening and crystallization. Fusion bonding of two such crystallizable amorphous polymer sheets in this processing temperature window is thus a complex process, different from fusion of purely amorphous polymer above T_g or semicrystalline polymer above T_m. In this study, the interfacial morphological development during fusion bonding of supercooled PET in the temperature window between T_g and T_m was studied. A unique double-zone interfacial morphology was observed at the bond. Transcrystals were found to nucleate at the interface and grow inward toward the bulk and appeared to induce nucleation in the bulk to form a second interfacial region. The size and morphology of the two zones were found to be significantly affected by the fusion bonding conditions, particularly the fusion temperature. The fusion bonding strength determined by the peeling test was found to be significantly affected by the state of crystallization and the morphological development at the bonding interface. Based on the interfacial morphology observed and the bonding strength measured, a fusion bonding mechanism of crystallizable amorphous polymer was proposed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

This two-article sequence on rubber-assisted embossing was aimed to understand the basic mechanisms affecting the pattern uniformity and replicability and to determine a process window for achieving uniform patterning and faithful replication. In Part I, the effects of major process and material parameters were identified and studied, and strategies for successful hot embossing with rubber as a pressure medium were proposed. In Part II, the rubber-assisted embossing process was analyzed considering the unique rheological behavior of the materials involved to develop useful predictive capabilities for this new process. Specifically, a finite-strain hyperelastic formulation was used for simulating the isothermal embossing stage, and a generalized Maxwell model was used to study the stress relaxation during the holding stage and to predict the elastic recovery after demolding. The rheological properties obtained in Part I were fitted to the constitutive models and implemented in the simulation procedures. The simulation results agreed with the major findings in the experimental work and provided a more quantitative insight into the process. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

In rubber-assisted hot embossing, a softened thin thermoplastic film is pressurized between a hard mold surface and a rubber pad. The rubber pad, as a soft counter-tool, deforms conformably to the hard mold surface, allowing feature transfer from the hard surface and formation of shell-type structures on the polymer film. This two-article sequence was aimed to understand the basic mechanisms affecting the pattern uniformity and replication in rubber-assisted hot embossing. In Part I, a series of rubber-assisted embossing experiments involving parametric studies of the effects of different processing conditions, as well as material selections, on the pattern thickness uniformity and replicated pattern height were conducted. The difference in film thickness uniformity in different experiments was explained using the mechanical and rheological behavior of the polymer film and the rubber counter-tool under different processing conditions. Based on the experimental results, strategies for determining a feasible process window for achieving uniform shell patterning by rubber-assisted embossing were proposed. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Processing-related properties of a polypropylene (PP)/silicone oil blend were investigated. It was found that an addition of a small amount (∼2 wt%) of silicone oil, a low molecular weight linear poly(dimethyl siloxane) (PDMS), drastically changed the rheological properties of PP. The PDMS seemed to work both as an internal lubricant and an external lubricant in the blend system. In particular, the apparent viscosity of the blend in capillary rheometry was approximately 10 times lower than that of virgin PP. The local minima in the viscosity versus shear rate curve in capillary rheometry and the gap-dependency of viscosity in parallel-plate rheometry implied the existence of a slip wall condition, caused by the formation of a thin lubricant layer at die walls. Thermal analyses and mechanical tests showed that the thermal and mechanical properties of the blend were nearly unaffected by the minor addition of silicone oil. The blend was tested in a profile extrusion process, and a significant reduction in die swell and profile distortion was achieved. The jet stretchability or spinnability in fiber spinning was also greatly improved with the minor addition of silicone oil. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

The hot embossing process has so far been developed mainly for replication of surface structures on thermoplastic substrates. Because of the lack of a through-thickness action, fabrication of discrete microparts such as microgears is considered difficult. In this study, embossing molds having multiple microcavities were used in a through-thickness embossing process with a rubber-assisted ejection mechanism. Microparts made of HDPE and ABS with each part weighing approximately 1 and 1.4 mg, respectively, were produced. When in the mold, embossed microparts were intermittently connected to each other through thin residual films of a thickness approximately 20 μm. The residual films were detached from the microparts during a rubber-assisted ejection stage. Because no resin delivery paths, e.g., runners and gates, are needed for microcavities on the multicavity embossing mold, this micropart fabrication process could replace micro injection molding in many applications. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers