Richard P. Chartoff

Interfacial characteristics of composites fabricated by laminated object manufacturing

Abstract This paper examines interfacial issues that arise when fabricating ceramic (SiC/SiC) and polymer matrix (glass/epoxy) composites using a novel, fully automated rapid prototyping method called Laminated Object Manufacturing (LOM). Discussed in this report are the three types of interfaces that are of concern in composites fabricated with the LOM system: interlaminar, particle–matrix in particulate composites, and fiber–matrix in fiber-reinforced composites. In order to obtain good interlaminar adhesion, it was necessary to address three issues: material preforms, LOM processing parameters, and high-temperature post-processing techniques. Developments in each of these areas led to the successful production of high-performance composites using the LOM process.

Development of a curved layer LOM process for monolithic ceramics and ceramic matrix composites

A novel rapid prototyping technology incorporating a curved layer building style was developed. The new process, based on laminated object manufacturing (LOM), was designed for efficient fabrication of curved layer structures made from ceramics and fiber reinforced composites. A new LOM machine was created, referred to as curved layer LOM. This new machine uses ceramic tapes and fiber prepregs as feedstocks and fabricates curved structures on a curved‐layer by curved‐layer basis. The output of the process is a three‐dimensional “green” ceramic that is capable of being processed to a seamless, fully dense ceramic using traditional techniques. A detailed description is made of the necessary software and hardware for this new process. Also reviewed is the development of ceramic preforms and accompanying process technology for net shape ceramic fabrication. Monolithic ceramic (SiC) and ceramic matrix composite (SiC/SiC) articles were fabricated using both the flat layer and curved layer LOM processes. For making curved layer objects, the curved process afforded the advantages of eliminated stair step effect, increased build speed, reduced waste, reduced need for decubing, and maintenance of continuous fibers in the direction of curvature.

Polymerization and viscoelastic behavior of networks from a dual‐curing, liquid crystalline monomer

The network formation and viscoelastic behavior of a liquid crystalline monomer, whose structure includes both acrylate and acetylene reactive groups, have been studied. By combining both photo and thermal polymerization, the networks can be formed in two separate steps, with the initial photopolymerization dominated by acrylate crosslinking and subsequent thermal polymerization dominated by acetylene crosslinking. In addition, the monomer exhibits a liquid crystalline phase. Photopolymerization while in the liquid crystal phase locks in the molecular ordering. Dynamic mechanical analysis shows that networks formed from the liquid crystalline phase have lower crosslink densities and narrower distributions of molecular weights between crosslinks when compared to networks formed from the isotropic phase (and at higher polymerization temperatures). After thermal postcure at 250°C, the networks formed from the isotropic monomer have a 23% higher dynamic mechanical storage modulus (in the glassy state) than the networks formed from the liquid crystalline monomer. The thermally postcured networks have unusually high glass-transition temperatures, which exceed 300°C.

Infrared spectral changes with crystallization in poly(vinylchloride): Correlations with X-ray and density data

Abstract The influence of crystallinity and stereoregularity on the infrared (IR) spectrum of atactic PVC in the solid state has been studied by many researchers [1-12]. Although the molecules in commercial PVC consist of both syndiotactic and isotactic sequences, the bulk polymer is not highly stereoregular, having approximately 50% syndiotacticity. Its infrared spectrum is different from that of highly syndiotactic PVC [3,5,7,9,10-12], particularly in the carbon-to-chlorine stretching region where there are three bands located at 610(615), 635, and 690 cm−1. These three bands are known to be of complex origin, since each band consists of more than one absorption frequency and its relative intensity depends on the physical state or history of the specimen [3,5,7,9,10-12]. The spectrum in this region is most rigorously interpreted in terms of chain conformational structure, the spatial arrangement of the atoms around the C-C1 bond. Thus, while changes in absorbance intensities for the bands with history d...

Photopolymerization of nematic liquid crystal monomers for structural applications : Linear viscoelastic behavior and cure effects

This article is concerned with molecular orientation in liquid crystal (LC) monomers and the retention of orientation in crosslinked network polymers formed from them by photopolymerization. This is of importance because anisotropic mechanical and physical properties can be beneficial in certain structural applications. To this end, linear viscoelastic behavior of liquid crystal photo-monomers was investigated with dynamic mechanical analysis, and molecular order was studied by infrared dichroism measured with Fourier transform infrared spectroscopy. Although the order parameter of the monomer could vary from 0.45 to 0.70, depending on temperature, the order parameters of the polymer samples varied only from 0.50 to 0.62 and depended on polymerization temperature and extent of cure. The mechanical anisotropy was found to be a complicated phenomenon that depended not only on the molecular order, but also on other factors such as free volume and network structure. The difference in the elastic modulus parallel and perpendicular to the alignment direction was as high as a factor of two in the glassy state, and a factor of three above Tg. In addition, different amounts of mechanical anisotropy could be induced by varying the cure conditions. Finally, different postcuring schemes could cause variations in mechanical behavior by advancing cure or by inducing secondary reactions.

Determination of the effects of cure conditions on the photopolymerization of liquid crystalline monomers using differential photo-calorimetry

Photopolymerization of diacrylate liquid crystalline monomers has several advantages over thermal polymerization. Most notably, photopolymerization allows for the cure reaction to occur at a temperature where a mesophase exists. In this paper we discuss the effects of several cure conditions, including temperature, on the conversion and reaction rates of two liquid crystalline monomers. Each of these monomers exhibits a nematic mesophase at elevated temperatures. The data indicate that the presence of the mesophase has an impact on both reaction rates and conversion. Further studies concerning the effects of photoinitiator composition and concentration reveal that these parameters significantly influence the polymerization rate but do not affect the degree of conversion at extended irradiation times.

Novel liquid crystal resins for stereolithography – processing parameters and mechanical analysis

The build characteristics of two liquid crystal (LC) reactive monomers were studied using a table‐top stereolithography apparatus (TTSLA). LC materials contain stiff, rod‐like mesogenic segments in their molecules, which can be aligned causing an anisotropy in properties. When cured in the aligned state the anisotropic structure is “locked in” resulting in materials with anisotropic physical and mechanical properties. By varying the alignment of layers, properties such as thermal expansion coefficient can be optimized. High heat distortion (or glass transition) temperatures are possible depending on the monomer chemical structure. Working curves for the LC resins were developed under various conditions. A permanent magnet placed outside the TTSLA vat was used to uniformly align the monomer in the nematic state. Photo‐initiator type and content; alignment of the nematic phase; and operating conditions affected the working curve parameters. Glass transition temperatures of post‐cured parts ranged from 75 to 1488C depending on the resin and processing conditions. Mechanical analysis data revealed a factor of two difference between glassy moduli measured in the molecular alignment versus the transverse alignment directions. Based on these initial studies, more advanced resins with higher glass transitions are being developed at the University of Dayton.

Thermal-expansion and fracture toughness properties of parts made from liquid crystal stereolithography resins

Abstract Stereolithography is a layered manufacturing technique that uses a laser to selectively polymerize a thermosetting resin to form complex three-dimensional parts. This rapid prototyping technique has evolved over the years from a model maker to a rapid fabrication technique for soft-tooling and parts for in-service testing. With this evolution in applications has come increased demand for high-performance resins. Liquid crystal (LC) resins can be processed by stereolithography methods to provide parts with superior and in some cases unique properties compared with conventional resins. LC resins consist of rod-like molecules that can be aligned prior to cure resulting in anisotropic mechanical and physical properties. A magnet outside the resin vat can be used to control the molecular alignment. Alignment can be varied within a layer or from layer-to-layer to achieve desired properties much as is done with fiber reinforced plastics. Two properties that are important for many applications are toughness and thermal dimensional-stability. Initial studies showed that fracture toughness values of LC coupons were about twice that of isotropic coupons manufactured from the same resin. Also, we have demonstrated that the in-plane thermal expansion of LC parts can be minimized over a wide temperature range by rotating the molecular alignment 90° from one layer to the next.

Compatibility of tire elastomers using derivative heat flow traces in the glass transition region

Derivative heat flow curves give much more information about the phase heterogeneity of binary blends composed of NR, SBR and BR elastomers thanTg. In blend compositions, the areas under the derivative heat flow curves appear to be an additive function of the concentration of elastomers in the case of incompatible blends (NR/BR, NR/SBR). They are less than additive for either a partially compatible blend (uncured SBR/BR) or a compatible blend (covulcanized SBR/BR). In the case of 60/40 SBR/BR blends, a DSC (T0.5) reveals a singleTg, in conformity with the earlier investigators, whereas the derivative heat flow curve shows two peaks (Tp) indicating incomplete homogenization of the phases. This is a new observation not mentioned in the published literature. Thus, derivative heat flow traces are likely to provide a unique tool to determine compatibility of elastomers. The study also reveals the importance of sample contact with the DSC pan in quantitative determinations.

Birefringence thermal analysis of liquid crystalline monomers and their photopolymers

Birefringence analysis was used to measure thermal transitions in liquid crystal diacrylate monomers and their corresponding polymers. In this technique, linearly polarized light was used to probe the sample as the temperature increased via a linear ramp. Various phase transitions were observed in the liquid crystalline monomers. In addition, the monomers were isothermally photopolymerized in the liquid crystalline state and the resulting polymer networks retained their liquid crystalline order. Glass-to-rubber transitions as well as indications of further thermal polymerization and stress relaxation were detected. Results from birefringence experiments were compared to those of more traditional thermal analysis techniques including DSC and TMA.