J. E. Mark

Polymer-Based Molecular Composites

This book discusses the development of polymeric materials over the last 30 years. The chemists ability to manipulate structure and tailor the properties of organic materials for specific applications is presented. The phenomenal success of polymer chemistry is addressed. The need for a research base in multiphase polymeric materials similar to that which underlies single-phase organics is examined.

Synthesis, structure, mechanical properties, and thermal stability of some polysulfone/organoclay nanocomposites

Abstract Polysulfone/organoclay nanocomposites were prepared via a solution dispersion technique, and were characterized by X-ray diffraction, transmission electron microscopy, stress–strain measurements in elongation, and thermogravimetric analysis. The X-ray and microscopy results demonstrated that at least at some compositions, the technique employed was successful in exfoliating and widely dispersing the clay platelets. The other measurements demonstrated considerable improvements in strength and modulus, and in thermal stability.

Synthesis, structure, and properties of hybrid organic–inorganic composites based on polysiloxanes. I. Poly(dimethylsiloxane) elastomers containing silica

Various synthetic protocols were used to prepare several classes of poly- siloxane-silica filler systems. The structures of these fillers and their interactions with the polysiloxane matrices were studied using small-angle X-ray and neutron scattering. In addition, the mechanical properties of the composites were characterized using equi- librium stress-strain isotherms in elongation. The results indicated that manipulation of the chemical reactions used to generate the filler can lead to a wide range of complex structures and unusual properties. Some of the observed mechanical properties were correlated with information on the composite structures and on elastomer-filler inter- actions. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1167-1189, 1998

Model networks of end‐linked polydimethylsiloxane chains. VII. Networks designed to demonstrate non‐Gaussian effects related to limited chain extensibility

End‐linking polymer chains by means of a multifunctional cross‐linking agent provides an ideal way for obtaining elastomeric networks of any desired distribution of chain lengths. In the present investigation, this technique was employed to give polydimethylsiloxane networks consisting of various proportions of relatively long and very short network chains. The stress–strain isotherms of these networks generally showed an anomalous increase in the modulus at high elongation, and the increase persisted at high temperatures and high degrees of swelling. This non‐Gaussian effect was quantitatively correlated with the limited extensibility of the network chains; specifcally, the increase in modulus was found to begin at approximately 60%–70% of the maximum extensibility of the network chains, and network rupture at 80%–90%. The elongation at which the increase becomes evident increases with decrease in the proportion of the very short chains, thus verifying the nonaffine nature of the deformation at high elon...

Preparation and properties of hybrid organic-inorganic composites prepared from poly(phenylene terephthalamide) and titania

Abstract The sol—gel process was used to prepare a class of composites in which a high-temperature polymer, poly(phenylene terephthalamide), was reinforced with varying amounts of in-situ generated titania. The polymer was synthesized by reacting a mixture of p - and m -phenylene diamines with terephthaloyl chloride in dimethylacetamide, using stoichiometry yielding chains with carbonyl chloride end groups. These chain ends were then replaced with methoxy groups using aminophenyltrimethoxysilane, and a titania network generated which should be chemically bonded to the polymer matrix through the hydrolysis of appropriate proportions of tetrapropylorthotitanate and water. The resulting composite films had amounts of titania ranging from 2.5 to 40 wt%, and were characterized with regard to their mechanical and thermal properties. The films containing relatively small amounts of titania were transparent and tough, and had tensile strengths the order of 193 MPa (relative to the 147 MPa of the pure copolymer). Thermal decomposition temperatures were in the range 350–450°C, and the weights of the samples remaining after heating to 800°C were found to be roughly proportional to the titania contents. Water absorption of the films consisting of pure Aramid was rather high (12.8 wt%), but decreased with increased amounts of titania. Dynamic mechanical thermal analysis showed a systematic increase in the glass transition temperature with increase in titania content. Increased amounts of titania also caused the tan δ peaks to shift to higher temperatures and to became broader and weaker, indicating the extent to which the mobility of the polymer chains was diminished by the titania phase.

A non‐Gaussian theory of rubberlike elasticity based on rotational isomeric state simulations of network chain configurations. II. Bimodal poly(dimethylsiloxane) networks

Bimodal, poly(dimethylsiloxane) (PDMS) networks containing a large mole fraction of very short chains have been shown to be unusually tough elastomers. The purpose of this investigation is to understand the rubber elasticity behavior of these bimodal networks. As a first approach, we have assumed that the average chain deformation is affine. This deformation, however, is partitioned nonaffinely between the long and short chains so that the free energy is minimized. Gaussian statistics are used for the long chains. The distribution function for the short chains is found from Monte Carlo calculations. This model predicts an upturn in the stress‐strain curve, the steepness depending on the network composition, as is observed experimentally.

A non‐Gaussian theory of rubberlike elasticity based on rotational isomeric state simulations of network chain configurations. I. Polyethylene and polydimethylsiloxane short‐chain unimodal networks

The present theoretical approach to rubberlike elasticity is novel in that it utilizes the wealth of information which rotational isomeric state theory provides on the spatial configurations of chain molecules. Specifically, Monte Carlo calculations based on the rotational isomeric state approximation are used to simulate spatial configurations, and thus distribution functions for the end‐to‐end separation r of the chains. Results are presented for polyethylene (PE) [CH−2] and polydimethylsiloxane (PDMS) [Si(CH3)2–O–] chains most of which are quite short, in order to elucidate non‐Gaussian effects due to limited chain extensibility. Large values of r were found to be more prevalent in PDMS than in PE, primarily because of the unusually large Si–O–Si bond angle in the PDMS chain, which increases its spatial extension. The use of these distribution functions in place of the Gaussian function for network chains gives upturns in modulus at high elongations, because of the rapidly diminishing number of configu...

Comparisons among the reinforcing effects provided by various silica-based fillers in a siloxane elastomer

Abstract Filled networks of poly(dimethyl siloxane) (PDMS) were prepared using the following wide variety of materials and techniques: (1) incorporating a commercial silica which had been treated with hexamethyldisilazane; (2) incorporating ‘wet-process’ silica which had been precipitated from a silicate in an aqueous dispersion with PDMS; (3) precipitating silica directly into PDMS during its curing; (4) precipitating silica directly into a swollen PDMS network after it was cured; (5) incorporating silica prepared from tetraethoxysilane (TEOS) and containing some PDMS, and (6) incorporating silica prepared from partially hydrolysed TEOS and also containing some PDMS. The resulting filled elastomers showed the largest values of the ultimate strength in the case of methods (4) and (6), and the largest value of the rupture energy for method (4).

Microcellular foams from polyethersulfone and polyphenylsulfone: Preparation and mechanical properties

Abstract Polymeric foams having microcellular structures were successfully prepared from some high-performance thermoplastics, specifically polyethersulfone and polyphenylsulfone. A two-stage batch foaming process was used and the resulting materials had average cell sizes in the range 2–13 μm, and cell densities the order of 10 10 –10 11 cells/cm 3 . The foam densities (relative to those of the unfoamed polymers) were in the range 0.90–0.35. Average cell sizes increased with foaming temperature and foaming time; on the other hand, cell densities and relative foam densities decreased slightly with foaming temperature but remained almost constant with foaming time. Experimental values of Young’s modulus in compression and the elastic collapse strength were higher than theoretically predicated at high relative densities, but the discrepancies became small at lower densities. In contrast, Young’s moduli in tension were in very good agreement with theory, but the relative strengths were somewhat lower than predicated.

Molecular dynamics simulation of diffusion of small molecules in polymers. II. Effect of free volume distribution

The influence of the free volume distribution on the diffusion of small molecules in polymers is studied by means of molecular dynamics simulations. A model is used which is composed of 10 O2 molecules, and 600 CH2 groups each belonging to an infinite length chain or its periodic images. A fully vibrational treatment is applied to the chains with a torsional potential and nonbonded interactions. Four model systems, each consisting of chains having the same molecular parameters except for the equilibrium skeletal bond angle, are compared at 300 K and at densities giving the same free volume fraction. The self‐diffusion coefficient of oxygen strongly correlates with the average interchain separation; a tight chain packing reduces the diffusion even when the total free volume content is kept constant. The importance of the free volume distribution for diffusion was demonstrated by the good correlation obtainable in terms of the effective free volume available between the nearest neighbor chains.